Which Operations Are Allowed

TypeScript knows what each operator does with each type, and it reports the operations that don't make sense. This program has four of them:

let gold = 120;
const priceTag = "15";

console.log(priceTag * 2);
console.log(gold - priceTag);
if (gold === "120") {
    console.log("Kira is rich!");
}
if (gold > "100") {
    console.log("Kira is rich!");
}

main.ts(4,13): error TS2362: The left-hand side of an arithmetic operation must be of type 'any', 'number', 'bigint' or an enum type.
main.ts(5,20): error TS2363: The right-hand side of an arithmetic operation must be of type 'any', 'number', 'bigint' or an enum type.
main.ts(6,5): error TS2367: This comparison appears to be unintentional because the types 'number' and 'string' have no overlap.
main.ts(9,5): error TS2365: Operator '>' cannot be applied to types 'number' and 'string'.

• Arithmetic needs numbers. The operators -, *, / and % are arithmetic operations, and both of their sides, the left-hand one and the right-hand one, must be numbers. (any turns the check off, and you'll get to it at the end of this lesson; bigint is a type for huge whole numbers, and enums are an older feature of TypeScript that this course doesn't use.) In JavaScript, "15" * 2 is 30, because JavaScript turns the text into a number by itself. TypeScript treats it as a mistake: you probably didn't know that the value is a text, and with +, the same value would join instead of add.
• A number and a text are never equal. === compares the types too, so in JavaScript, gold === "120" is always false. TypeScript tells you that the comparison looks unintentional, because no value can be a number and a text at once (the types have “no overlap”). Comparing with < or > is an error too.
• Joining with + is allowed. When one side of + is a text, + joins, as you know: "Gold: " + gold.

When a number is stored as text, convert it. Number() from the JavaScript course turns a text into a number, and String() turns a number into a text:

console.log(Number("15") * 2);
console.log(String(15) + 2);

This prints:

30
152

TypeScript knows that Number(…) is always a number, so Number(priceTag) * 2 is fine. It doesn't know whether the text really contains a number, though: Number("fifteen") gives NaN, as in JavaScript.