Variables Without a Value

A variable created without a value, such as let reward: number;, has no value at first: in JavaScript, it holds undefined. The value is stored in it later, often in the branches of an if. TypeScript follows every path through the program and checks that wherever you use the variable, it surely has a value:

let monster = "goblin";
let reward: number;

if (monster === "wolf") {
    reward = 50;
} else if (monster === "goblin") {
    reward = 20;
}

console.log("Reward:", reward);

main.ts(10,24): error TS2454: Variable 'reward' is used before being assigned.

The message says that the variable is used before a value is assigned to it. But the monster is a goblin, so reward gets 20, doesn't it? This time, yes. TypeScript doesn't look at what monster holds right now, though. It checks every path the program could take. If the monster were a dragon, neither branch would run, and the program would print Reward: undefined. A calculation such as gold + reward would then give NaN.

To fix it, make sure that every path stores a value, most often with an else branch, which runs in all the other cases:

if (monster === "wolf") {
    reward = 50;
} else if (monster === "goblin") {
    reward = 20;
} else {
    reward = 0;
}

Another way is a starting value: let reward = 0;. The variable then has a value from the start, the branches only change it, and you don't even need the type, because TypeScript infers it from the 0. Choose what fits: a starting value when there is a sensible one, and a variable without a value when the value really comes only from the branches. TypeScript then checks that you haven't forgotten any of them.

Typical mistakes:

• An if or else if without a final else.
• A branch that does something else and forgets to store the value.
• Using a variable that never gets a value at all: let reward: number; followed by console.log(reward); gives the same TS2454.