Value returned by the assignment

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That’s the way the language was designed. It is consistent with most languages.

Having a variable declaration return anything other than undefined is meaningless, because you can’t ever use the var keyword in an expression context.

Having assignment be an expression not a statement is useful when you want to set many variable to the same value at once:

x = y = z = 2;

It can also be used like this:

x = 2*(y = z); // Set y = z, and x = 2*z

However that is not the most readable code and it would probably be better written as:

y = z;
x = 2*z;

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