Exact behaviour of == and === operator in JavaScript

Question

While writing simple snippet for comparison in JavaScript I observed some weird behavior.

Case 1:

typeof(window.WHTStatement.DDL_TPTypeID.size()) ==> "number"
typeof(window.WHTStatement.Txt_TPTypeValue.size()) ==> "number"

window.WHTStatement.DDL_TPTypeID.size() == 1 == window.WHTStatement.Txt_TPTypeValue.size()

returns true -- OK

Case 2:

window.WHTStatement.DDL_TPTypeID.size() === 1 == window.WHTStatement.Txt_TPTypeValue.size()

returns true -- OK

Case 3:

window.WHTStatement.DDL_TPTypeID.size() === 1 === window.WHTStatement.Txt_TPTypeValue.size()

returns false, why?

What exactly happening here in case 3. Can somebody elaborate?

It's not "weird" behaviour; there is a difference between "weird" behaviour and behaviour which you -- not anyone else, though! -- did not expect. — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Nov 4, 2011 at 7:38

ThiefMaster · Accepted Answer · 2011-11-04 07:33:29Z

4

Unlike Python, in JS x == y == z is not equal to x == y && y == z but (x == y) == z. So you are actually comparing a boolean to a number which obviously fails in a type check.

The == comparison worked because 1 == true is true.

answered Nov 4, 2011 at 7:33

ThiefMaster

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