--- id: 645cbb5ab1296e49946adb6e title: Step 47 challengeType: 0 dashedName: step-47 --- # --description-- In computer science, a stack is a data structure where items are stored in a LIFO (last-in-first-out) manner. If you imagine a stack of books, the last book you add to the stack is the first book you can take off the stack. Or an array where you can only `.push()` and `.pop()` elements. The call stack is a collection of function calls stored in a stack structure. When you call a function, it is added to the top of the stack, and when it returns, it is removed from the top / end of the stack. You'll see this in action by creating mock call stack. Initialize a variable named `callStack` and assign it an empty array. # --hints-- You should create a variable named `callStack`. ```js assert.exists(callStack); ``` You should assign `callStack` an empty array. ```js assert.isArray(callStack); assert.isEmpty(callStack); ``` # --seed-- ## --seed-contents-- ```html Decimal to Binary Converter

Decimal to Binary Converter

``` ```css *, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; } :root { --light-grey: #f5f6f7; --dark-blue: #1b1b32; --orange: #f1be32; } body { background-color: var(--dark-blue); font-family: "Times New Roman", Times, serif; font-size: 18px; color: var(--light-grey); padding: 0 15px; display: flex; flex-direction: column; align-items: center; } h1 { text-align: center; font-size: 2.3rem; margin: 20px 0; } .input-container { margin: 10px 0; display: flex; flex-direction: column; gap: 10px; justify-content: center; align-items: center; } .convert-btn { background-color: var(--orange); cursor: pointer; border: none; padding: 4px; } .number-input { height: 25px; } #result { margin: 10px 0; min-width: 200px; width: fit-content; min-height: 80px; word-break: break-word; padding: 15px; border: 5px solid var(--orange); font-size: 2rem; text-align: center; } #animation-container { margin: auto; max-width: 300px; } .animation-frame { margin: 250px auto 0; padding: 15px 10px; border: 5px solid var(--orange); font-size: 1.2rem; text-align: center; } @media screen and (min-width: 500px) { .input-container { flex-direction: row; } #result { max-width: 460px; } } ``` ```js --fcc-editable-region-- --fcc-editable-region-- const a = () => { return "freeCodeCamp " + b(); }; const b = () => { return "is " + c(); }; const c = () => { return "awesome!"; }; console.log(a()); const numberInput = document.getElementById("number-input"); const convertBtn = document.getElementById("convert-btn"); const result = document.getElementById("result"); const decimalToBinary = (input) => { let binary = ""; if (input === 0) { binary = "0"; } while (input > 0) { binary = (input % 2) + binary; input = Math.floor(input / 2); } result.innerText = binary; }; const checkUserInput = () => { if ( !numberInput.value || isNaN(parseInt(numberInput.value)) || parseInt(numberInput.value) < 0 ) { alert("Please provide a decimal number greater than or equal to 0"); return; } decimalToBinary(parseInt(numberInput.value)); numberInput.value = ""; }; convertBtn.addEventListener("click", checkUserInput); numberInput.addEventListener("keydown", (e) => { if (e.key === "Enter") { checkUserInput(); } }); ```