The postfix and prefix forms

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The postfix and prefix forms

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What are the final values of all variables a, b, c and d after the code below?

let a = 1, b = 1;
let c = ++a; // ?
let d = b++; // ?

The answer is:

a = 2
b = 2
c = 2
d = 1

let a = 1, b = 1;
alert( ++a ); // 2, prefix form returns the new value
alert( b++ ); // 1, postfix form returns the old value
alert( a ); // 2, incremented once
alert( b ); // 2, incremented once