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I was reading about Javascript operators precedence over here and got curious why I can't write something like this: 

let num = 1;
++num++;

Which gets Uncaught ReferenceError: Invalid left-hand side expression in prefix operation error. But why is that? :)

Aurimas
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1 Answers1

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It evaluates as

++(num++)

so, the expression

num++

returns a number, not the variable, because it is a primitive value. The added plusses, throws an exception, because a primitive value is not a variable and an assigment is not possible.

Nina Scholz
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  • Yes, you are right! Just `num++` is evaluated first, but the essence remains the same. Thanks, my curiosity needs satisfied! :) – Aurimas May 17 '17 at 11:19