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JavaScript === vs == : Does it matter which “equal” operator I use?

In some JavaScript if statements I’ve seen the use of === as opposed to the standard ==.

What is the difference between these? and should I be using either one of them over the other?

e.g.

if (variable == 'string') {
 return;
}

compared to:

if (variable === 'string') {
 return;
}

Answer 1

The === is a strict comparison (also checks type), whereas the == does a more relaxed comparison.

For instance:

var a = 'test';
if (a == true) {
   // this will be true
}

if ( a === true) {
   // this will not be true
}

Another example:

var b = '0';
if ( b == 0){
    // this will be true
}

if ( b === 0 ){
    // this will not be true
}

In particular it is very important when comparing falsy values. In Javascript all the following will be treated as false with relaxed comparison:

* false
* null
* undefined
* empty string ''
* number 0
* NaN