JavaScript conditions and array loops

Filter values with strict comparisons and accumulate a total.

On this page

Before you start

const, let, and arrays.

Filter values with strict comparisons and accumulate a total.
Predict the sample output and explain the state changes.
Complete the practice task and check the boundary cases.

The concept

for...of visits array values. let allows the accumulator binding to change, while const keeps the array binding fixed.

A const array can still have mutable elements; const prevents rebinding, not deep mutation. This example does not modify the input array.

Code example

JavaScript

const values = [-2, 0, 3, 5];
let total = 0;
for (const value of values) {
  if (value > 0) total += value;
}
console.log(total);
if (total !== 8) throw new Error("Unexpected total");

Output

Example details

Follow the code

Open the walkthrough to inspect the example one step at a time.

Show guided walkthrough

Guided walkthroughReady

1const values = [-2, 0, 3, 5];

2let total = 0;

3for (const value of values) {

4 if (value > 0) total += value;

5}

6console.log(total);

7if (total !== 8) throw new Error("Unexpected total");

Follow the example

Start to inspect the source, values, and output. These prepared steps explain the published sample; grouped loop steps describe multiple iterations.

View the complete dry-run table

Prepared trace of the published program. Some rows group repeated iterations.
What happens	State after step	Output so far
Create four numeric values.	`values = [-2, 0, 3, 5]`	—
Initialize the accumulator.	`total = 0`	—
Skip -2 and 0, then add 3.	`total = 3`	—
Add 5.	`total = 8`	—
Print the result.	`total = 8`	8

Behind the scenes

How JavaScript processes this code

A JavaScript engine parses and runs the source. A function body runs when the function is called. console.log writes to the console.

Cost and common mistakes

O(n) time and O(1) auxiliary space.

for...in enumerates property keys, not array values.
Number arithmetic cannot exactly represent every large integer; validate ranges when needed.

Try it yourself

Change the program to count positive values rather than sum them.

Reveal answer and reasoning

Increment total by 1 for every positive value; the result is 2.