Originally published on php-to-js.learnio.dev — part of the PHP to JavaScript course. View this lesson on the original site.

let and const stay inside the block where you declare them

In PHP, a variable declared inside an if or for is still visible in the rest of the function. JavaScript let and const stop at the closing }.

PHP habit

<?php
function firstItem(array $items): ?string {
    if (count($items) > 0) {
        $first = $items[0];
    }
    return $first ?? null;
}

JavaScript equivalent

function firstItem(items) {
  let first = null;
  if (items.length > 0) {
    const candidate = items[0];
    first = candidate;
  }
  return first;
}

Situation	PHP	JavaScript (`let`/`const`)
Variable inside `if`	Visible after the block	Not visible outside
Loop counter with `let`	N/A	New binding each iteration
Legacy `var` in JS	Leaks like PHP	Avoid in new code

Declare outside when you need the value later

function process(items) {
  let status = 'empty';
  if (items.length > 0) {
    status = 'ok';
  }
  return status;
}

Same pattern as initialising $status before an if in PHP.

Loops and closures

const fns = [];
for (let i = 0; i < 3; i++) {
  fns.push(() => i);
}
console.log(fns.map((fn) => fn())); // [0, 1, 2]

// with var — all closures share one i → [3, 3, 3]

Use let in loops when callbacks capture the index — covered again with closures next lesson.

Try it

Use the amber Try it panel below. Click Run.

function blockDemo() {
  const outer = 'outside';
  if (true) {
    const inner = 'inside block';
    console.log(outer, inner);
  }
  console.log('after block:', outer);
  // console.log(inner); // ReferenceError
}
blockDemo();

You should see two lines inside the block and one after — no access to inner outside.

From PHP to JavaScript — Control Flow, Functions, and Scope