Why this expression is var a = 3, b = a = typeof b; giving undefined?

Question

This expression is giving me the undefined output in this console. Please help to understand why it is printing undefined?

var a = 3, b = a = typeof b; // undefined

Because the console evaluates the expression, and it does not have anything to return, so it prints `undefined`. — TechySharnav, Mar 27 '21 at 06:37

score 2 · Accepted Answer · answered Mar 27 '21 at 06:36

2

What really happens here is:

var a = 3;    // so a gets a value.
var b;        // var b exists but is undefined.
a = typeof b; // a receives type of b, which is undefined at this time
b = a;        // b gets the value from a

answered Mar 27 '21 at 06:36

Kevin Verstraete

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I got it. Thank you Kevin for your explanation. :) – yogesh kumar Mar 27 '21 at 06:47

score 1 · Answer 2 · answered Mar 27 '21 at 06:37

An assignment operator assigns a value to its left operand based on the value of its right operand. That is, a = b assigns the value of b to a.

Thus, in our case

b = a = typeof b;

a gets the value undefined and then b.

var a=3, b = a = typeof b;
undefined
a
"undefined"
b
"undefined"

Why this expression is var a = 3, b = a = typeof b; giving undefined?

2 Answers2