JavaScript Type Conversion Practice Programs with Solutions (31 Programs)

console.log(Number('42'));
console.log(Number('3.14'));
console.log(Number(''));

Output

42
3.14
0

Explanation: Number() converts a whole string to a number. An empty string becomes 0.

console.log(Number('42px'));
console.log(Number('abc'));

Output

NaN
NaN

Explanation: Number() returns NaN if any part of the string is not numeric.

console.log(parseInt('42px', 10));
console.log(parseFloat('3.75 kg'));
console.log(parseInt('px42', 10));

Output

42
3.75
NaN

Explanation: the parse functions read digits from the start and stop at the first invalid character. If the string does not start with a number, they return NaN.

const code = 404;
const a = String(code);
const b = code.toString();
const c = `${code}`;
console.log(a, b, c);
console.log(typeof a, typeof b, typeof c);

Output

404 404 404
string string string

Explanation: String(), .toString() and template literals all produce a string.

console.log(Boolean(0));
console.log(Boolean(1));
console.log(Boolean(''));
console.log(Boolean('0'));
console.log(Boolean('false'));

Output

false
true
false
true
true

Explanation: any non-empty string is truthy, even '0' and 'false'.

const quantity = '25';
console.log(quantity + 5);
console.log(+quantity + 5);

Output

255
30

Explanation: without conversion + joins strings, giving '255'. Unary + converts the string first.

console.log('5' + 3);
console.log('5' - 3);
console.log('5' * '2');
console.log('10' / '2');

Output

53
2
10
5

Explanation: + prefers string concatenation, while -, * and / convert both sides to numbers.

const amount = 19.987;
const asString = amount.toFixed(2);
const asNumber = Number(asString);
console.log(asString, typeof asString);
console.log(asNumber, typeof asNumber);

Output

19.99 string
19.99 number

Explanation: toFixed returns a string. Wrap it with Number() when you need a number again.

const tags = ['smoke', 'regression'];
console.log(String(tags));
console.log(tags.join(' | '));

Output

smoke,regression
smoke | regression

Explanation: arrays convert to comma-separated strings. join lets you choose the separator.

const csv = 'chromium,firefox,webkit';
const browsers = csv.split(',');
console.log(browsers);
console.log(browsers.length);

Output

[ 'chromium', 'firefox', 'webkit' ]
3

Explanation: split converts a delimited string into an array of strings.

console.log(0 == '');
console.log(0 === '');
console.log('1' == 1);
console.log(null == 0);

Output

true
false
true
false

Explanation: == converts types before comparing, with some surprising rules. Prefer === in tests.

console.log(parseInt('1010', 2));
console.log(parseInt('ff', 16));
console.log((255).toString(16));

Output

10
255
ff

Explanation: the second argument of parseInt is the base. toString(base) converts the other way.

console.log(Number('12.9'));
console.log(parseInt('12.9', 10));
console.log(Math.trunc(Number('12.9')));

Output

12.9
12
12

Explanation: parseInt drops the decimal part. Use Number to keep it, or Math.trunc for an explicit integer.

const config = { browser: 'chromium', retries: 2, headless: true };
const json = JSON.stringify(config);
console.log(json, typeof json);
const parsed = JSON.parse(json);
console.log(parsed.retries + 1);

Output

{"browser":"chromium","retries":2,"headless":true} string
3

Explanation: JSON.stringify turns data into text, for files or APIs. JSON.parse turns it back into objects with real numbers and booleans.

const result = { test: 'login', status: 'passed' };
console.log(JSON.stringify(result, null, 2));

Output

{
  "test": "login",
  "status": "passed"
}

Explanation: the third argument of JSON.stringify adds indentation, which makes logs easier to read.

const data = { a: undefined, b: () => 1, c: NaN, d: 'ok' };
console.log(JSON.stringify(data));

Output

{"c":null,"d":"ok"}

Explanation: JSON drops undefined and functions, and turns NaN into null.

const date = new Date('2026-01-15T10:30:00Z');
console.log(date.getTime());
console.log(date.toISOString());
console.log(new Date(0).toISOString());

Output

1768473000000
2026-01-15T10:30:00.000Z
1970-01-01T00:00:00.000Z

Explanation: getTime() gives milliseconds since 1 January 1970 UTC, and toISOString() gives a standard text format.

const price = {
  amount: 499,
  valueOf() { return this.amount; },
  toString() { return `₹${this.amount}`; }
};
console.log(price + 1);
console.log(`${price}`);

Output

500
₹499

Explanation: maths calls valueOf, while template literals call toString.

console.log(isNaN('abc'));
console.log(Number.isNaN('abc'));
console.log(Number.isNaN(Number('abc')));

Output

true
false
true

Explanation: global isNaN converts its argument first. Number.isNaN only returns true for the actual NaN value.

const scores = { login: 95, checkout: 80 };
const map = new Map(Object.entries(scores));
console.log(map.get('checkout'));
const back = Object.fromEntries(map);
console.log(back);

Output

80
{ login: 95, checkout: 80 }

Explanation: Object.entries and Object.fromEntries convert between plain objects and key/value pairs.

const priceText = '₹1,299.50';
const price = Number(priceText.replace(/[^0-9.]/g, ''));
console.log(price);
console.log(price + 100);

Output

1299.5
1399.5

Explanation: remove currency symbols and thousands separators, then convert. UI text is always a string.

const priceTexts = ['$12.50', '$7.25', '$30.00'];
const total = priceTexts
  .map(text => Number(text.replace('$', '')))
  .reduce((sum, value) => sum + value, 0);
console.log(total.toFixed(2));

Output

49.75

Explanation: summing the raw strings would concatenate them, so convert each to a number first.

const env = { WORKERS: '4' };
const workers = Number(env.WORKERS ?? 2);
const retries = Number(env.RETRIES ?? 0);
console.log(workers, typeof workers);
console.log(retries);

Output

4 number
0

Explanation: environment variables are always strings or missing. Convert them and provide defaults.

const toBool = value => ['true', '1', 'yes'].includes(String(value).toLowerCase());
console.log(toBool('true'));
console.log(toBool('false'));
console.log(toBool('1'));
console.log(toBool(undefined));

Output

true
false
true
false

Explanation: Boolean('false') is true, so compare against known true values instead.

const line = 'qa_user,25,true';
const [username, age, active] = line.split(',');
const user = { username, age: Number(age), active: active === 'true' };
console.log(user);

Output

{ username: 'qa_user', age: 25, active: true }

Explanation: CSV fields are text, so convert each column to the type your test expects.

const apiPrice = 1299;
const uiText = '1,299';
console.log(apiPrice === uiText);
console.log(apiPrice === Number(uiText.replace(/,/g, '')));

Output

false
true

Explanation: normalise the UI text into a number before comparing it with API data.

const durationMs = 125430;
const totalSeconds = Math.floor(durationMs / 1000);
const minutes = Math.floor(totalSeconds / 60);
const seconds = totalSeconds % 60;
console.log(`${minutes}m ${String(seconds).padStart(2, '0')}s`);

Output

2m 05s

Explanation: integer division with Math.floor and % converts a raw number into a readable duration.

const summary = 'Passed 45 of 50 tests in 12.5 seconds';
const numbers = summary.match(/\d+(\.\d+)?/g).map(Number);
console.log(numbers);
console.log(numbers[0] / numbers[1]);

Output

[ 45, 50, 12.5 ]
0.9

Explanation: match with a global regular expression finds every number as a string, and map(Number) converts them.

const inputs = [5, '7', '', null, '3'];
const quantities = inputs.map(value => (value === '' || value == null ? 0 : Number(value)));
console.log(quantities);
console.log(quantities.reduce((a, b) => a + b, 0));

Output

[ 5, 7, 0, 0, 3 ]
15

Explanation: decide explicitly how empty values convert, rather than relying on implicit coercion.

const amount = 1234567.891;
console.log(amount.toLocaleString('en-US', { style: 'currency', currency: 'USD' }));
console.log(amount.toLocaleString('en-IN', { style: 'currency', currency: 'INR' }));

Output

$1,234,567.89
₹12,34,567.89

Explanation: toLocaleString converts numbers into locale-aware text, which is useful when asserting formatted prices.

const params = new URLSearchParams('?page=2&sort=price&inStock=true');
const query = {
  page: Number(params.get('page')),
  sort: params.get('sort'),
  inStock: params.get('inStock') === 'true'
};
console.log(query);

Output

{ page: 2, sort: 'price', inStock: true }

Explanation: URLSearchParams returns every value as a string, so convert numbers and booleans yourself.