Lesson

JS Arrays

Q: How do I empty a JavaScript array?

If using let , assign items = [] . If you must keep the same const reference shared elsewhere, set items.length = 0 .

Q: What happens when I read an index that does not exist?

Reading items[99] on a 3-item array returns undefined instead of throwing an error.

10 min reading

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JavaScript Arrays: Ordered Lists and Zero-Based Indexing

A JavaScript array is an ordered, resizable list that stores multiple values inside a single variable. Each element sits at a numeric index starting at 0, and arrays can hold strings, numbers, objects, or even nested arrays.

Creating and Reading Arrays

Create arrays with square bracket literals [...] rather than new Array(). Bracket literals are shorter and avoid the confusing single-number trap where new Array(5) creates five empty slots instead of [5].

Read elements from the start with items[0] or from the end with items.at(-1) (ES2022). Checking items.length returns the total count of slots in the array.

Practical Array Basics Example

Using spread syntax [...arr] lets you copy or combine arrays without changing the original lists.

const languages = ["HTML", "CSS", "JavaScript"];

// Read first and last elements cleanly
console.log("First:", languages[0]);
console.log("Last:", languages.at(-1));

// Update by index and copy with spread
languages[1] = "Tailwind CSS";
const fullStack = [...languages, "Node.js", "PostgreSQL"];

console.log("Count:", fullStack.length);
console.log("Full stack:", fullStack);
console.log("Is Array:", Array.isArray(fullStack));

Common Mistakes to Avoid

Using typeof arr to detect arrays: typeof ["a"] returns "object". Always use Array.isArray(value) to confirm an array.
Creating sparse holes with high indexes: Assigning list[10] = "x" on a 2-item array leaves 8 undefined empty slots in between. Use push() or spread syntax instead.
Removing items with delete arr[1]: delete leaves an undefined hole and keeps .length unchanged. Use splice() or filter().