Formatting templating strings in ES6

formatting.js

function format(strings, ...values) {
  const pieces = [];
  for (let [index, string] of strings.entries()) {
    let value = values[index];

    const formatIndex = string.lastIndexOf('$');
    if (formatIndex != -1) {
      const format = string.substr(formatIndex + 1);
      string = string.substr(0, formatIndex);
      // TODO: Use an actual format engine here.
      switch (format) {
        case '02d':
          if (value < 10) value = '0' + value;
          break;
        case 'json':
          value = JSON.stringify(value);
          break;
      }
    }

    pieces.push(string);
    if (value !== undefined) pieces.push(value);
  }
  return pieces.join('');
}

const person = {name: 'Lisa', occupation: 'Engineer'};
console.log(format`${person.name}'s information: $json${person}`);
// -> Lisa's information: {"name":"Lisa","occupation":"Engineer"}
const minutes = 3, seconds = 47;
console.log(format`The eggs will be ready in $02d${minutes}:$02d${seconds}.`);
// -> The eggs will be ready in 03:47.