for Loop

A for loop repeats a block using an initializer, a condition and an update expression.

In this lesson, you will study the rule, run a complete TypeScript example, explain its output, and test a small change.

The standard form is for (initialization; condition; update) { ... }. The initializer runs once, the condition is checked before each iteration, and the update runs after each completed body. Carefully choosing start and end conditions prevents off-by-one errors.

Use the diagram to locate the checks, values or branches involved in the example.

Save this code as main.ts or paste it into the TypeScript Playground. The example uses fixed values so it runs without prompting for input.

TypeScript code

for (let i = 1; i <= 3; i++) {
   console.log(`Step ${i}`);
 }

Verified output

Step 1
Step 2
Step 3

How to run locally: npx tsc --strict --target es2022 --module commonjs main.ts, then node main.js. When using a project configuration, run the matching project build instead.

The loop starts at i = 1 and prints Step 1. Each update increments i. It prints Step 2 and Step 3, then stops when i becomes 4.

Predict the output before you run the sample. Compare each line with the source expression that produced it, and distinguish the type checker’s work from what happens in JavaScript at runtime.

Do not confuse i < 3 with i <= 3: the former excludes 3. Ensure the update moves toward the stopping condition.

Debugging habit: when a change is rejected, read the exact compiler diagnostic and inspect the operand types. When it compiles but behaves unexpectedly, follow the values through the JavaScript expressions.

Change the loop to print from 5 down to 1. Decide on the condition and decrement before running.

Challenge: write down your prediction first, run the changed source, and explain any difference. Return to the verified version before continuing.

for Loop: The standard form is for (initialization; condition; update) {.

The update expression follows each iteration before the next condition check.

Before continuing, be able to explain what is checked before execution and what actually changes at runtime in the example above.