Which equals operator (== vs ===) should be used in JavaScript comparisons?

The strict equality operator (===) behaves identically to the abstract equality operator (==) except no type conversion is done, and the types must be the same to be considered equal. Reference: JavaScript Tutorial: Comparison Operators The == operator will compare for equality . The === operator will do the conversion, so if two values are not the same type === will simply return false. Both are equally quick. To quote Douglas Crockford's excellent JavaScript: The Good Parts,

JavaScript has two sets of equality operators: === and !==, and their evil twins == and !=. The good ones work the way you would expect. If the two operands are of the same type and have the same value, then === produces true and !== produces false. The evil twins do the right thing when the operands are of the same type, but if they are of different types, they attempt to coerce the values. The rules by which they do that are complicated and unmemorable. These are some of the interesting cases:

'' == '0'           // false
0 == ''             // true
0 == '0'            // true

> ```
false == 'false'    // false
false == '0'        // true

false == undefined  // false
false == null       // false
null == undefined   // true

> ```
' \t\r\n ' == 0     // true

The lack of transitivity is alarming. My advice is to never use the evil twins. Instead, always use === and !==. All of the comparisons just shown produce false with the === operator.

Update

A good point was brought up by @Casebash in the comments and in @Phillipe Laybaert's answer concerning objects. For objects, == and === act consistently with one another (except in a special case).

var a = [1,2,3];
var b = [1,2,3];

var c = { x: 1, y: 2 };
var d = { x: 1, y: 2 };

var e = "text";
var f = "te" + "xt";

a == b            // false
a === b           // false

c == d            // false
c === d           // false

e == f            // true
e === f           // true

The special case is when you compare a primitive with an object that evaluates to the same primitive, due to its toString or valueOf method. For example, consider the comparison of a string primitive with a string object created using the String constructor.

"abc" == new String("abc")    // true
"abc" === new String("abc")   // false

Here the == operator is checking the values of the two objects and returning true, but the === is seeing that they're not the same type and returning false. Which one is correct? That really depends on what you're trying to compare. My advice is to bypass the question entirely and just don't use the String constructor to create string objects from string literals.

https://262.ecma-international.org/5.1/#sec-11.9.3

In JavaScript, there are two equality operators: the loose equality operator (==) and the strict equality operator (===). The main difference between them is that the loose equality operator performs type coercion before comparing the values, while the strict equality operator compares both the values and their types without any type coercion.

In terms of performance, using the strict equality operator (===) is generally faster than using the loose equality operator (==). This is because the strict equality operator does not perform type coercion, which can be an additional overhead.

Here's an example to illustrate the difference:

console.log(1 == '1');  // true (loose equality)
console.log(1 === '1'); // false (strict equality)

In the first comparison (1 == '1'), the loose equality operator coerces the string '1' to a number before comparing it with the number 1. This type coercion takes some time, although it may be negligible in most cases.

In the second comparison (1 === '1'), the strict equality operator directly compares the number 1 with the string '1' without any type coercion. Since their types are different, the comparison returns false.

When comparing values of the same type, both == and === will have similar performance. However, using === is still recommended as it avoids unexpected type coercion and promotes more precise comparisons.

In your example, idSele_UNVEHtype.value.length === 0 is preferred over idSele_UNVEHtype.value.length == 0 because it explicitly compares the length with the number 0 without any type coercion.

While the performance difference between == and === may not be significant in most cases, using === is considered a good practice for several reasons:

1. It promotes code clarity and reduces the chances of unexpected behavior due to type coercion.
2. It catches potential bugs early by ensuring that the compared values have the same type.
3. It aligns with the principle of "explicit is better than implicit," making the code more maintainable and easier to understand.

Therefore, it is generally recommended to use the strict equality operator (===) for comparisons in JavaScript, unless you specifically need the type coercion behavior of the loose equality operator (==).

The choice between using the == (loose equality) or === (strict equality) operator in JavaScript depends on the desired behavior and coding best practices, rather than performance considerations.

Loose Equality (==) vs. Strict Equality (===):

The == operator performs type coercion, which means that it attempts to convert the operands to a common type before comparing their values. This can lead to unexpected results, especially when comparing values of different types. For example:

console.log(1 == '1'); // true
console.log(0 == false); // true
console.log(null == undefined); // true

The === operator, on the other hand, performs a strict comparison without any type coercion. It checks if the operands are of the same type and have the same value. For example:

console.log(1 === '1'); // false
console.log(0 === false); // false
console.log(null === undefined); // false

Performance Considerations:

In terms of performance, the difference between == and === is negligible. Both operators perform equally well, and the performance impact is insignificant in most cases.

Best Practices:

Despite the potential performance implications being negligible, it is generally recommended to use the === operator for comparisons in JavaScript. Using === helps avoid unexpected behavior caused by type coercion and makes the code more explicit and easier to understand.

Here are some best practices regarding the use of == and ===:

1. Use === by default: Always use === unless you have a specific reason to use ==.
2. Avoid type coercion: Type coercion can lead to unexpected behavior and should be avoided unless it is intentional and well-documented.
3. Consistent use: Choose either == or === and use it consistently throughout your codebase.
4. Exceptions: There are a few exceptions where == can be used, such as when checking for null or undefined values, as null == undefined evaluates to true, while null === undefined evaluates to false.

In summary, while there is no significant performance difference between == and ===, it is recommended to use === for strict equality comparisons in JavaScript to avoid unexpected behavior caused by type coercion and to follow best coding practices.

In JavaScript, == (loose or abstract equality comparison) and === (strict or identity comparison) have different use cases.

== performs type coercion when comparing values. This means it tries to convert both sides of the comparison operator into a common type before doing the comparison. For instance, if you compare a string "5" with the number 5 using ==, they will be considered equal because JavaScript converts the string to a number.

===, on the other hand, does not perform type coercion and only checks for equality based on their data types and values (identity). For instance, it would not consider "5" and 5 as equal if comparing with ===.

Regarding your question about performance benefits, JavaScript engines are highly optimized these days, and the difference in performance between using == or === is insignificant. Using === can help avoid unintended type conversions, making your code clearer and easier to understand.

Therefore, using === consistently throughout your codebase would be a good practice for ensuring expected equality behavior and reducing potential confusion in future maintenance or enhancement tasks.

The choice between using the == (loose equality) operator and the === (strict equality) operator in JavaScript comparisons is primarily a matter of coding style and ensuring code correctness, rather than a significant performance consideration.

Here's a breakdown of the differences and considerations:

1. Type Conversion:

   • The == operator performs type coercion, meaning it will try to convert the operands to a common type before making the comparison.
   • The === operator, on the other hand, is a strict equality comparison that does not perform type coercion. It checks both the value and the type of the operands.

2. Performance Impact:

   • The performance difference between using == and === is generally negligible in modern JavaScript engines. The time it takes to perform the comparison is insignificant compared to other operations in your code.
   • In most cases, the performance impact of using == or === is not a significant factor in your code's overall performance.

3. Code Correctness:

   • The === operator is generally preferred because it provides more predictable and reliable results, especially when dealing with values of different types.
   • Using === can help you avoid unexpected behavior that can arise from type coercion, which can sometimes lead to bugs in your code.

4. JSLint Recommendations:

   • JSLint is a code quality tool that enforces certain coding style guidelines, including the recommendation to use === instead of ==.
   • The rationale behind this recommendation is to promote code correctness and readability, rather than to address performance concerns.

In summary:

• The === operator is generally preferred over == because it provides more predictable and reliable results, especially when dealing with values of different types.
• The performance difference between using == and === is negligible and should not be a primary concern when choosing between the two.
• The main reason to replace == with === is to improve code correctness and readability, as recommended by tools like JSLint.

If no type conversion is taking place in your comparisons, there would be no performance gain in using === over ==. However, using === is still generally considered a better practice for maintaining code quality and avoiding potential issues.

Performance Impact:

In most cases, there is no significant performance difference between using == and === in JavaScript. Modern JavaScript engines optimize both operators equally.

Type Conversion vs. Strict Comparison:

• == (loose equality) performs type conversion before comparison, allowing different types to be compared.
• === (strict equality) performs a strict comparison without type conversion, requiring the operands to be of the same type.

When to Use ===:

It is generally recommended to use === for strict comparisons, especially when comparing values that may have different types. This ensures that the comparison is accurate and prevents unexpected results due to type conversion.

When to Use ==:

• When you intentionally want to perform type conversion.
• When you are comparing values that are guaranteed to be of the same type.

Example:

// Use === for strict comparison
if (name === "John Doe") {
  // ...
}

// Use == for type conversion (not recommended)
if (age == 30) {
  // ...
}

Conclusion:

While === is generally preferred for strict comparisons, there is no significant performance benefit over == in most cases. However, using === ensures accuracy and prevents unexpected type conversion issues.

Performance Difference:

While the === operator is generally considered the faster of the two, the difference in performance is typically negligible in modern JavaScript environments.

According to the MDN Web Docs:

• === performs strict equality check (value and type)
• == performs value equality check (only for value types)

Therefore, the performance improvement is usually not worth the effort of replacing == with ===.

Type Conversion:

Yes, the === operator performs type conversion before performing the equality check. This means that if the operands are of different types (e.g., one is a number and the other is a string), === will first convert them to the same type before doing the comparison.

Conclusion:

While the === operator is generally faster, the performance difference is usually negligible. Therefore, unless you have specific performance concerns or profiling evidence, it's not necessary to replace == with ===.

Additional Considerations:

• The === operator can be used for type coercion, but this can have side effects in some cases.
• The == operator may still be necessary when comparing objects, as they are not strictly comparable.

Great question! In JavaScript, both == (abstract equality) and === (strict equality) are used for comparisons, but they behave differently.

The primary difference between the two is that == performs type coercion, trying to convert and compare the values, while === does not. This leads to various "gotcha" moments when using ==, because of its implicit type conversion.

To answer your question about performance, using === is indeed faster than == because it doesn't involve type coercion. But the difference in performance is usually negligible, and you should prioritize writing clear and maintainable code over micro-optimizations.

Here's a quick benchmark to demonstrate the difference:

const benchmark = (f, num) => {
  const start = Date.now();
  for (let i = 0; i < num; i++) {
    f();
  }
  return Date.now() - start;
};

const strictComparison = () => {
  let a = 5;
  let b = 5;
  a === b;
};

const looseComparison = () => {
  let a = 5;
  let b = 5;
  a == b;
};

console.log('Strict Comparison: ', benchmark(strictComparison, 1000000));
console.log('Loose Comparison: ', benchmark(looseComparison, 1000000));

However, when comparing values of different types, === is generally recommended. While it might not matter much for simple scripts, it can prevent subtle bugs that are hard to track down in larger applications.

In your specific example, using idSele_UNVEHtype.value.length === 0 would be better as it checks if the length is strictly equal to zero. Nevertheless, if you are using == and the code works as expected, you don't necessarily have to change it for performance reasons. But, as a best practice, using === is a safer choice.

The strict equality operator (===) behaves identically to the abstract equality operator (==) except no type conversion is done, and the types must be the same to be considered equal. Reference: JavaScript Tutorial: Comparison Operators The == operator will compare for equality . The === operator will do the conversion, so if two values are not the same type === will simply return false. Both are equally quick. To quote Douglas Crockford's excellent JavaScript: The Good Parts,

JavaScript has two sets of equality operators: === and !==, and their evil twins == and !=. The good ones work the way you would expect. If the two operands are of the same type and have the same value, then === produces true and !== produces false. The evil twins do the right thing when the operands are of the same type, but if they are of different types, they attempt to coerce the values. The rules by which they do that are complicated and unmemorable. These are some of the interesting cases:

'' == '0'           // false
0 == ''             // true
0 == '0'            // true

> ```
false == 'false'    // false
false == '0'        // true

false == undefined  // false
false == null       // false
null == undefined   // true

> ```
' \t\r\n ' == 0     // true

The lack of transitivity is alarming. My advice is to never use the evil twins. Instead, always use === and !==. All of the comparisons just shown produce false with the === operator.

Update

A good point was brought up by @Casebash in the comments and in @Phillipe Laybaert's answer concerning objects. For objects, == and === act consistently with one another (except in a special case).

var a = [1,2,3];
var b = [1,2,3];

var c = { x: 1, y: 2 };
var d = { x: 1, y: 2 };

var e = "text";
var f = "te" + "xt";

a == b            // false
a === b           // false

c == d            // false
c === d           // false

e == f            // true
e === f           // true

The special case is when you compare a primitive with an object that evaluates to the same primitive, due to its toString or valueOf method. For example, consider the comparison of a string primitive with a string object created using the String constructor.

"abc" == new String("abc")    // true
"abc" === new String("abc")   // false

Here the == operator is checking the values of the two objects and returning true, but the === is seeing that they're not the same type and returning false. Which one is correct? That really depends on what you're trying to compare. My advice is to bypass the question entirely and just don't use the String constructor to create string objects from string literals.

https://262.ecma-international.org/5.1/#sec-11.9.3

It's worth noting that using === is recommended in many programming languages for comparing objects instead of relying on the == operator because it's safer and more reliable due to JavaScript's lack of object-oriented types.

In terms of performance, there should be no significant difference between using == or ===, especially when dealing with simple types such as strings or numbers. However, for objects that are compared by reference (i.e., instances of custom classes), using === will prevent errors due to different implementations of equality.

Overall, it's a good practice to use === whenever possible in JavaScript comparisons, especially when dealing with complex data structures.

In an SEO analysis software application, there is an array of websites being analyzed, and for each website, we store some specific characteristics like the domain type ('dom' or 'com') and the type of traffic ('organic', 'direct') among other details. The applications are coded in JavaScript using ES5 (ES6 on modern browsers), with == operator mostly used.

Assume you have a list of these websites: [ { "url": "http://domain1.com", "traffic": "organic"} { "url": "http://domain2.org", "traffic": "direct" } ]

We also know that if two elements (objects) in our list are identical, they are not really the same object. The only exception is when you have a custom object type like so: class MyClass { ...} where all of its fields are initialized with 'value'. Here's an instance of such a custom object:

function MyObject() { this.type = "custom"; this.id = 0; // just for comparison purposes only, no need to assign it on this class return this; } var myObj1 = new MyClass(); var myObj2 = new MyObject(); myObj2.id++; myObj1 === myObj2 // Returns false. Even though they're equal according to == operator, but not same objects since id property is different

Let's assume you are using ES6 features and can use == for object comparison as per your requirement: [ { "url": "http://domain1.com", "traffic": "organic"} { "url": "http://domain2.org", "traffic": "direct" } ]

and now you're going to replace all the == comparisons in your application with === for better readability and safety: [ { "url": "http://domain1.com", "traffic": "organic"} { "url": "http://domain2.org", "traffic": "direct" } ]

However, you run into a problem during testing – your code starts giving runtime errors! After some investigation, it's revealed that in the custom object class (MyClass), all fields except for id were set to 'value', but when two objects of this class are compared with == operator, only id is considered.

Question: In your JavaScript application, can you modify your comparison method such that all properties are considered in comparing two instances, and not just the first property found during iteration (id property)? How would you go about this?

First, consider using a technique called 'Proof by Exhaustion', which is where we try out each possible solution until the correct one has been found. In JavaScript, when using the forEach() or each() methods, it's necessary to iterate over every property in an object (except for id). In this case, our custom MyClass has multiple properties: 'type', 'id' and a few others that are never used in comparison. It is evident that we need to traverse each of these properties during the comparison operation.

To modify your existing code, you would need to add a custom method called getObjectPropertyValues() within your MyClass which takes two parameters: self (the current instance), and prop (the property whose value to return). It should look something like this:

MyClass.prototype._getObjectPropertyValues = function(obj, prop) {
  var values;
  for (key in obj) if (!obj.hasOwnProperty(prop)) continue; // Skip properties not owned by instance
  values = key in obj && obj[key]; 
  return [values]
}

The method returns an array with the current property value for each object, regardless of its type (custom or native). To use it within your comparison logic, simply modify it to check if any property value returned is not 'value'. For instance:

if(Object.keys(_.getObjectPropertyValues(myObj1, "type")[0]) != ["custom"]) return false; // Excluding the id field that we know from MyObjects are identical in custom objects.

This should be effective for comparing instances of your custom object with any other JavaScript objects as well, not just each other. It's a demonstration of using tree-of-thought reasoning and proof by exhaustion to find an elegant solution to a problem.

Answer: The answer is no, the '==' comparison operator can't directly compare two custom objects without explicitly considering their id fields. This requires writing your own logic which checks every property for equality, similar to what has been explained in step 2.

=== vs. == in JavaScript Comparisons

The debate between === and == boils down to type coercion and performance.

=== Operator:

• Checks for exact equality, including both value and type.
• No type conversion happens.
• More strict and avoids pitfalls of == like type coercion and the unexpected.
• Can be more readable and prevent bugs.

== Operator:

• Checks for value equality, ignoring the type.
• Type conversion may occur, which can have performance implications.
• Can be confusing and lead to unexpected results due to type coercion.
• Can be less readable and prone to errors.

Performance Considerations:

While the strict equality check provided by === is beneficial for preventing bugs and ensuring type integrity, it comes at a cost. Comparing primitive types like numbers and strings using === can be slightly slower than == due to the additional type verification.

However, when objects or complex data structures are being compared, === offers a significant performance boost as it avoids unnecessary type conversions.

In Your Specific Case:

Given your example of idSele_UNVEHtype.value.length == 0 within an if statement, using === instead of == would be more appropriate as it ensures exact equality and avoids potential issues related to type conversion.

Recommendation:

• Use === when you want to ensure exact equality and avoid type coercion.
• Use == cautiously when comparing primitive types and performance is critical.

Additional Tips:

• If you're using JSLint, consider setting the eqeq option to true, which recommends using === instead of ==.
• Use tools like the console.time() function to benchmark and compare the performance of === and == in your specific code.

In conclusion:

While === is generally preferred for stricter equality comparisons and better readability, there are performance implications associated with its use. Consider the context and performance requirements of your code when deciding between == and ===.

Using === in your JavaScript code is a good habit to form. Using == can lead to errors in comparison as it has a lower precedence than other operators (it compares objects for equality before comparing values) and type conversions, which means that a == b does not mean the same thing as a === b.

If you are simply checking if two primitive types have equal value, using === will provide better type-safe behavior than ==. This can help avoid errors due to implicit conversion between primitives and other values.

There's no substantial performance improvement when you switch from == to ===. Both comparison operators perform an equal value check and the difference between them is only in how they handle type conversion.

In JavaScript, == performs a loose equality operation which involves implicit type coercion. This means that if two values are compared using ==, JavaScript tries its best to convert one or both of the operands into a common type before making comparison, often through automatic type conversion (e.g., converting numbers back and forth between different string representations).

On the other hand, === is known as a strict equality operator and does not do any type coercion. This means that if two values are compared using ===, their types must match in order for it to return true.

In terms of readability, generally speaking, developers recommend sticking to === whenever possible, because its explicitness can often lead to clearer code and avoid potential bugs or unintended effects related to type coercion. So using JSLint suggests you switch from == to === for these situations.

However, in some specific scenarios (e.g., when dealing with JavaScript's built-in objects like NaN and null/undefined), the behavior of == and === can differ because those are not strict equality operators but have their own unique behaviors due to language specification. So always ensure to understand these particular cases before making a decision for using == or ===.

In JavaScript, both == (two equals signs) and === (three equals signs) can be used for comparison. As far as performance, there is no clear difference between using == or ===. Both operators are evaluated at compile time by JavaScript engines like Chrome and Firefox. The difference in the output of the comparison is due to the fact that both == and === use strict equality (i.e., identity) when comparing object references, as well as using reference equality (i.e., identity) when comparing numeric values. Therefore, there is no clear performance benefit or disadvantage to using either == or === for comparison in JavaScript.