If functions in a programming language are treated similarly to other variables, the language is said to have first-class functions. Therefore, the functions can be returned by another function, assigned to any other variable, or supplied as an argument. Functions are treated as first-class citizens in JavaScript. This implies that functions are just another sort of object and are nothing more than values.

Let's use an illustration to learn more about the first-class function.

<script>
	const codesolutionstuff = {
		add: (x, y) => {
			return `${x} + ${y} = ${x + y}`;
		},
		subtract: (x, y) => {
			return `${x} - ${y} = ${x - y}`
		},
		multiply: (x, y) => {
			return `${x} * ${y} = ${x * y}`
		},
		division: (x, y) => {
			if (y != 0) return `${x} / ${y} = ${x / y}`;
			return `Cannot Divide by Zero!!!`;
		}

	}

	document.write(codesolutionstuff.add(200, 200) + "<br>");
	document.write(codesolutionstuff.subtract(100, 7) + "<br>");
	document.write(codesolutionstuff.multiply(50, 50) + "<br>");
	document.write(codesolutionstuff.division(100, 5));
</script>

Output:

200 + 200 = 400
100 - 7 = 93
50 * 50 = 2500
100 / 5 = 20

Example 2:

<script>
    const codesolutionstuff = (x, y) => {
        return (x + " " + y);
    }
      
    document.write(codesolutionstuff("Shailesh", "Coder"));
</script>

Output:

Shailesh Coder

First class functions are what it's all about, and we've better explained how they're used and implemented.