Nanfeng

Notes on software development, code, and curious ideas

TypeScript String Formatting with {0} and {1} Indexed Placeholders

In
Build a TypeScript format-string helper that replaces indexed placeholders such as {0} and {1} with variadic arguments while handling repeats and missing values.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
function formatString(str: string, ...args: unknown[]): string {
  return str.replace(/\{(\d+)\}/g, (match, indexText) => {
    const index = Number(indexText);
    return typeof args[index] !== 'undefined'
      ? String(args[index])
      : match;
  });
}

const name = 'Alice';
const age = 25;
const message = formatString(
  'My name is {0} and I am {1} years old.',
  name,
  age
);

console.log(message);
// My name is Alice and I am 25 years old.

The regular expression captures the numeric index inside each placeholder. The callback uses that index to retrieve the corresponding argument. If the argument is missing, the original placeholder remains unchanged.

+