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javascriptoperator-precedencetypeof

typeof of boolean expression with comparison operator

if (typeof foo !== 'undefined') {
    // Now we know that foo is defined, we are good to go.
}

The typeof evaluates to true or false based on whether the variable foo is defined or not.

But, say if foo !== 'undefined' evaluates to true, then typeof of true should evaluate to 'boolean'. Why does it evaluate to true or false?

Solution

  • Because precedence rules for the typeof and inquality operators define that that expression is parsed as

    (typeof foo) !== 'undefined'

    For more information, see the MDN page on operator precedence. typeof is precedence 16; inquality is precedence 10. The higher precedence of typeof means that it "binds" more tightly to its operand.

    By the way, why do you have undefined variables in your code?