Javascript ternary operator and assignment

Question

I get unexpected result for this simple JavaScript assignment statement:

var t = 1 == 1 ? 1 : 0;
undefined

I would have expected to get 1 assigned to t instead. Same result if you do

var t = (1 == 1 ? 1 : 0);
undefined

Can somebody explain why this does not work as expected?

Answer 1

The result of evaluating var t = 1 == 1 ? 1 : 0; in, say, the Firebug console will be undefined. However, the value of t will be 1 as expected. Try outputting t after the assignment.

Firebug will print the result when the variable declaration is on a separate line:

var t;
t = 1 == 1 ? 1 : 0;

This is because the return value of an assignment operation is the value being assigned. However, when the var keyword is present, what's returning is the value of the VariableStatement declaration, which behaves as follows:

The production VariableStatement : var VariableDeclarationList; is evaluated as follows: Evaluate VariableDeclarationList. Return (normal, empty, empty).

Where Return (normal, empty, empty). refers to a type recognized by JavaScript internally, not something that would be printed to the console.

Further reading:

http://ecma262-5.com/ELS5_HTML.htm#Section_12.2