var

You will not write var, but you will read it. Every JavaScript file older than about 2016 uses it, so it is worth twenty minutes to recognise what it does.

Function scope, not block scope

let and const live inside the nearest braces. var ignores blocks entirely and lives in the whole function.

function check() {
  if (true) {
    var inside = 'leaked';
  }
  return inside;
}

console.log(check());

leaked

The same code with let throws a ReferenceError, which is the more useful answer.

Hoisting

A var declaration is moved to the top of its function before anything runs. The assignment stays where it was, so the name exists early holding undefined.

function show() {
  console.log(value);
  var value = 'set later';
  console.log(value);
}

show();

undefined
set later

No error, just undefined, which is exactly the kind of bug that survives to production. let makes the same code throw.

The classic loop bug

Because there is one var for the whole function rather than one per pass, everything that captures it ends up sharing the final value.

var jobs = [];

for (var i = 0; i < 3; i++) {
  jobs.push(function () { return i; });
}

console.log(jobs.map(function (job) { return job(); }));

[3, 3, 3]

All three functions read the same i, and by the time they run the loop has finished and left it at 3. Change var to let and the answer becomes [0, 1, 2].

Redeclaration is allowed

Declaring the same var twice is legal and silent, so two unrelated pieces of code can share a name by accident.

var total = 1;
var total = 2;

console.log(total);

2