The number type represents numeric values in JavaScript. Integers and floating-point values use the same built-in type, so a variable can hold values such as 12 or 100 without a separate integer type. section 3.3.1
Use number values carefully when JavaScript combines them with other types:
- Adding a number to a numeric string can concatenate the values instead of calculating a total. section 3.2
- The loose comparison between 100 and "100" succeeds, even though one value is a number and the other is a string. section 3.2
- JavaScript infers a value's type dynamically, so assigning 100 to a variable previously holding a string changes the reported type to number. section 3.3.1
A numeric value mixed with text
let hatPrice = 100; let bootsPrice = "100"; let totalPrice = hatPrice + bootsPrice; console.log(totalPrice); // 100100 console.log(typeof hatPrice); // number The comparison may report equal, but the addition concatenates the number and string. A function expecting numbers can similarly produce NaN for "Hello" or 1.2 for true. section 3.2 section 7.2
Listing 3.6 Adding statements in the index.ts file in the primer folder
let hatPrice = 100;
console.log(`Hat price: ${hatPrice}`);
let bootsPrice = "100";
console.log(`Boots price: ${bootsPrice}`);
if (hatPrice == bootsPrice) {
console.log("Prices are the same");
} else {
console.log("Prices are different");
}
let totalPrice = hatPrice + bootsPrice;
console.log(`Total Price: ${totalPrice}`);
Inspect the complete price example and its output to see how comparison and addition treat a number alongside a numeric string. section 3.2
