assignment in javascript and the var keyword

Question

I was reading the \'learning Node\' book and I was stuck in a very simple issue, one that I haven\'t given too much thought about: assignment in javascript.

The auth

Answer 1

Per ECMA-262 § 12.2, a VariableStatement (that is, var identifier=value) explicitly returns nothing. Additionally, a VariableStatement is a Statement; Statements do not return values, and it is invalid to put a Statement somewhere an Expression would go.

For example, none of the following make sense because they put a Statement where you need value-yielding Expressions:

var a = var b;
function fn() { return var x; }

Per § 11.13.1, assignment to a variable (identifier=value) returns the assigned value.

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When you write var a = 1;, it declares a and initalizes its value to 1. Because this is a VariableStatement, it returns nothing, and the REPL prints undefined.

a=1 is an expression that assigns 1 to a. Since there is no a, JavaScript implicitly creates a global a in normal code (but would throw a ReferenceError in strict mode, since you're not allowed to implicitly create new globals in strict mode).

Regardless of whether or not a existed before, the expression still returns the assigned value, 1, so the REPL prints that.

Answer 2

var a = 2 is a statement. Thus it has no value.

Answer 3

You are evaluating a statement list. When evaluating a statement list, the value of the last value-producing statement is returned. http://ecma-international.org/ecma-262/5.1/#sec-12.1 - note the examples at the end of this section. If no statements in the list returned a value, then nothing will be returned (this is undefined in JavaScript).

The variable statement, does not return a value. http://ecma-international.org/ecma-262/5.1/#sec-12.2

The assignment operator, does return a value (as well as performing the assignment). http://ecma-international.org/ecma-262/5.1/#sec-11.13.1

Answer 4

Just guessing here - this could probably be verified by referring to the ECMAScript 5th Edition spec (but man that thing is a pain) - it probably has to do with the specification of the "var" statement vs assigning attributes to the "global" object.

When you declare a variable and assign a value to it (var a=2) the returned value is likely "undefined" because that's what the spec says the "var" statement should return.

When you assign a variable to a symbol without using the "var" statement you are actually assigning a value to an attribute of the global object of that name. That is, a=2 is the same as saying window.a=2 and we know that assinging a value to an attribute returns the value assigned.