JS: null, undefined, and undeclared

Published on: October 6, 2014

Tags: js and interview-questions

What’s the difference between a variable that is: null, undefined, or undeclared?

Here’s a TL;DR version I found quite useful when writing this.

I think they make more sense in the opposite order: undeclared, undefined, and null. That’s the order I’m going to tackle them anyways.

First up is undeclared.

A variable is undeclared when it does not use the var keyword. It gets created on the global object (that is, the window), thus it operates in a different space as the declared variables.

Example:

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var declaredVariable = 1;

function scoppedVariables() {
  undeclaredVariable = 1;
  var declaredVariable = 2;
}

scoppedVariables();

undeclaredVariable; // 1
declaredVariable; // 1

For more discussion on undeclared variables, checkout Mozilla’s documentation.

Note: this will not work in strict mode. It will throw an error when it gets to the undeclaredVariable - see this example. Thanks to Glen Selle for his comment with this update.


Now let’s move on to undefined.

Something is undefined when it hasn’t been defined yet. If you call a variable or function without having actually created it yet the parser will give you an not defined error.

Example:

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var undefinedVariable; // undefined
typeof undefinedVariable; // "undefined"

undefinedFunction(); // undefined
typeof undefinedFunction; // "undefined"

Note that the typeof returns "undefined", therefore undefined is a primitive type.

The fix for an undefined variable or function is easy, simply define it:

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var definedVariable = 'test';
typeof definedVariable; // "string"

function definedFunction(){
  return "I'm defined!"
}
typeof definedFunction // "function"

You can know if a variable is undefined with the following:

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if (typeof(variable) !== "undefined") {
  console.log('variable is not undefined');
} else {
  console.log('variable is undefined');
}


Finally we'll finish up with null.

null is a variable that is defined to have a null value.

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var nullVariable = null; // null
typeof nullVariable // "object"

You probably don’t often purposefully define a variable to null, but it may be the return value of a function. Often you'll need to gaurd against null values in your code.

You can know if a variable is null with the following:

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if( variable === null ) {
  console.log('variable is null');
} else {
  console.log('variable is not null');
}


I think the order “undeclared, undefined, and null” makes sense since it’s increasing order of certainty.


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