How to fix Type 'string' is not assignable to type 'number' in TypeScript

// Before: Type error
let age: number = "25"; // Error: Type 'string' is not assignable to type 'number'

// After: Correct conversion
let age: number = Number("25"); // age = 25

// Works with decimals
let price: number = Number("19.99"); // price = 19.99

// Returns NaN for invalid inputs
let invalid: number = Number("abc"); // invalid = NaN

const input = "123";
const value = Number(input);

if (isNaN(value)) {
  console.error("Invalid number input");
} else {
  console.log("Valid number:", value);
}

// Parse decimal integers
let count: number = parseInt("42", 10); // count = 42

// Automatically removes decimal portion
let rounded: number = parseInt("42.7", 10); // rounded = 42

// Handles leading whitespace
let spaced: number = parseInt("  100", 10); // spaced = 100

// Can extract numbers from mixed strings
let mixed: number = parseInt("123abc", 10); // mixed = 123

// Good: explicit radix
parseInt("08", 10); // 8

// Bad: implicit radix (older JS versions might interpret as octal)
parseInt("08"); // Could be 0 in older environments

// Parse floating-point numbers
let temperature: number = parseFloat("98.6"); // temperature = 98.6

// Works with scientific notation
let scientific: number = parseFloat("1.23e-4"); // scientific = 0.000123

// Handles leading/trailing whitespace
let trimmed: number = parseFloat("  3.14  "); // trimmed = 3.14

parseInt("3.14", 10);  // Returns 3 (integer only)
parseFloat("3.14");    // Returns 3.14 (preserves decimal)

// Unary plus conversion
let value: number = +"123"; // value = 123

// Works with variables
const stringValue = "456";
let numValue: number = +stringValue; // numValue = 456

// Useful in inline expressions
const total: number = +price1 + +price2;

// React example
const handleSubmit = (event: React.FormEvent<HTMLFormElement>) => {
  const formData = new FormData(event.currentTarget);
  const age: number = +(formData.get("age") as string);
};

// Angular example
onSubmit() {
  const quantity: number = +this.form.value.quantity;
}

// Allow both types
let id: string | number;

id = "user-123";  // Valid
id = 123;         // Also valid

// Function accepting either type
function formatId(id: string | number): string {
  return `ID: ${id}`;
}

formatId("abc-123"); // Works
formatId(456);       // Also works

function processValue(value: string | number) {
  if (typeof value === "number") {
    // TypeScript knows value is number here
    return value.toFixed(2);
  } else {
    // TypeScript knows value is string here
    return value.toUpperCase();
  }
}

function safeNumberConversion(input: string): number | null {
  const num = Number(input);

  // Check if conversion resulted in valid number
  if (isNaN(num)) {
    return null;
  }

  return num;
}

// Usage
const userInput = "123";
const result = safeNumberConversion(userInput);

if (result !== null) {
  console.log("Valid number:", result);
} else {
  console.error("Invalid number input");
}

import { z } from "zod";

const NumberSchema = z.string().transform((val) => {
  const num = Number(val);
  if (isNaN(num)) {
    throw new Error("Invalid number");
  }
  return num;
});

// Safe parsing
const parsed = NumberSchema.safeParse("123");
if (parsed.success) {
  const value: number = parsed.data; // TypeScript knows this is a number
}