Values, Types, and Expressions

Type Conversion

Convert between strings and numbers explicitly.

type-conversion `Number(...)` turns a numeric string into a number and `String(...)` turns a number back into text, so values can be used in the right context.

Type Conversion

text
type_conversion.js
Replay: real traced execution (multi-file project)
const text = "42";
const num = Number(text);
const back = String(num);
const doubled = num * 2;

console.log("num=" + num);
console.log("back=" + back);
console.log("doubled=" + doubled);

const text = "7";
const num = Number(text);
const back = String(num);
const doubled = num * 2;

console.log("num=" + num);
console.log("back=" + back);
console.log("doubled=" + doubled);

const text = "100";
const num = Number(text);
const back = String(num);
const doubled = num * 2;

console.log("num=" + num);
console.log("back=" + back);
console.log("doubled=" + doubled);

  1. text ← 42, num ← 42, back ← 42, doubled ← 84

    1const text→ 42 = "42"; //@text="7", "100"2const num→ 42 = Number(text42);3const back→ 42 = String(num42);4const doubled→ 84 = num→ 42 * 2;56console.log("num=" + num42);7console.log("back=" + back42);8console.log("doubled=" + doubled84);
    outputnum=42
back=42
doubled=84

  1. text ← 7, num ← 7, back ← 7, doubled ← 14

    1const text→ 7 = "7";2const num→ 7 = Number(text7);3const back→ 7 = String(num7);4const doubled→ 14 = num→ 7 * 2;56console.log("num=" + num7);7console.log("back=" + back7);8console.log("doubled=" + doubled14);
    outputnum=7
back=7
doubled=14

  1. text ← 100, num ← 100, back ← 100, doubled ← 200

    1const text→ 100 = "100";2const num→ 100 = Number(text100);3const back→ 100 = String(num100);4const doubled→ 200 = num→ 100 * 2;56console.log("num=" + num100);7console.log("back=" + back100);8console.log("doubled=" + doubled200);
    outputnum=100
back=100
doubled=200

Follow the Conversion

  1. text starts as "42".
  2. Number(text) gives num = 42.
  3. String(num) gives back = 42.
  4. doubled = num * 2 becomes 84.
  5. The program prints num=42, back=42, and doubled=84. | text | num | back | doubled | | --- | --- | --- | --- | | "7" | 7 | 7 | 14 | | "42" | 42 | 42 | 84 | | "100" | 100 | 100 | 200 |

Exercise: type_conversion.js

Reproduce doubled=84, then use the pinned text values 7 and 100 to predict each doubled value.