Order of evaluation in if statement in Javascript

Question

In order to protect my code from accessing undeclared variables I use

if (typeof myVar != 'undefined')

This works fine but I'd like to stick another if statement in it. Something like converting this:

if (typeof myVar != 'undefined'){
    if (myVar == "test"){}
}

to this:

if (typeof myVar != 'undefined' && myVar == "test")

Considering myVar may be undefined, is this last code secure in every case of usage and every browser?

Is it possible that various statements inside an if () are not evaluated in the order they're written?

Can I assume myVar == "test" will never be executed if myVar is undefined?

Answer 1

Can I assume myVar == "test" will never be executed if myVar is undefined?

Yes. The expression you are testing is a logical AND expression and the order of evaluation of the two operands is specified:

The production LogicalANDExpression : LogicalANDExpression && BitwiseORExpression is evaluated as follows (emphasis added):

• Let lref be the result of evaluating LogicalANDExpression.
• Let lval be GetValue(lref).
• If ToBoolean(lval) is false, return lval.
• Let rref be the result of evaluating BitwiseORExpression.
• Return GetValue(rref).

That basically says evaluate the first operand, if the result of that evaluation is false, the entire expression is false, and the second operand is never evaluated.

Answer 2

Yes, it is OK.

In javascript, the precedence is as below:

typeof > !=/== > &&

For a && b, b will never be executed when a is false.