Help! The increment command consistently gives the wrong answer;

JavaScript
#1

Tell us what’s happening:

This basic section of Functional programming is confusing me.
When I set the incrementer seen below to return val + 1. I am returned the correct answer of 5 for newVal, and do not directly update the fixedValue; this falls apart for some reason if I set the return value to val++; I just keep returning 4, and I am not sure why. This issue has persisted across multiple lessons. Am I misusing it somehow?

Your code so far


// The global variable
var fixedValue = 4;

// Only change code below this line
let incrementer = (val) => val ++;


// Only change code above this line

let newVal = incrementer(fixedValue);
console.log(newVal)

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Challenge: Pass Arguments to Avoid External Dependence in a Function

Link to the challenge:

#2

Do you want to pre or post increment?

#3