if(a = 1) is true [duplicate]

This question already has an answer here:

• Variable assignment inside an 'if' condition in JavaScript 6 answers

I don't know if it's possible duplicate. Testing my code, sometimes I was wrong with because I put = and not == or === in if conditions:

Consider this code :

var a = 0;
if(a = 1) console.log('true'); 

I don't know why this is not an error and why returns true the condition (a = 1) I guess that what it does is assign 1 to the a variable, but why this evaluates to true and there's no error?