javascripttype-coercion

Why !+[]+[]+![] is 'truefalse'?

As the title says. I'm learning coercion in JavaScript and I don't understand the return of that expression.

const test = !+[]+[]+![];
console.log(test);

Solution

  • First you have to be aware of how the expression is evaluated so for expressions like this it could be useful to take a look on operator precedence

    I will wrap the expressions with parenthesis and see how it goes. The logical not precedes addition (infix +)

    !(+[])+([])+(!([]))

    !(0)+([])+false

    true+([])+false

    Then when you try to evaluate true+[] it will convert both to string and will result in "true", after that you will add "true"+false, false will be converted to string.