Global Variable in JavaScript , variables defined without var

Question

The function below will return b is not defined because the compiler will look in the function and then into the global scope in search for a variable b .

However, I assumed that defining b without the word var would automatically create a global variable?

Can anyone explain the rules when omitting the word var ?

function foo(a) {    
  console.log( a + b );    
  b = a;
}

foo( 2 );

Answer 1

Not using var in a function for a variable declaration does make it global, but in your case, the JavaScript engine is hitting this line:

console.log( a + b );

before it hits this line:

b = a;

And, that's the line that declares it (globally).

And, because you didn't use var , the declaration is not hoisted to the top of the code base (which it would have been with var - you would still not have gotten a value for b because only the declaration would be hoisted, not the initialization, but it would not have thrown an error), so you get your error.

See more about var and hoisting here .

Answer 2

In strict mode:

• Using an undeclared variable will throw an exception

In non-strict mode:

• Assigning to an undeclared variable will create a global, but this is not hoisted
  • Declaring a variable with var creates a local variable and is hoisted to the top of the function
• Reading an undeclared variable will throw an exception

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Since you are not in strict mode and you try to read b before you assign a value to it, you get an exception.

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Guidelines to follow:

• Always "use strict"
• Always declare your variables
• Declare them in the global scope if you want them there
• … but try to avoid global scope. Consider using a closure instead if you think a global would be useful.

Answer 3

Variables declared this way b = a are not hoisted, like variables declared with the var keyword. That means that, at runtime, the compiler reads b, not as undefined (as would happen with var b = a ), but as something that doesn´t exist at all, thus throwing a ReferenceError.

Info on Hoisting: https://developer.mozilla.org/en-US/docs/Glossary/Hoisting