How to use 'js to ts converter online' in JavaScript

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function jsToTs(code) {
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  const output = code
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    // Change `+x` to `readonly x`:
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    .replace(/(\n *)\+(\[?[_a-zA-Z]+)/g, '$1readonly $2')
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    // Fix differently-named types:
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    .replace(/\bmixed\b/g, 'unknown')
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    .replace(/\| void\b/g, '| undefined')
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    // Fix `import type` syntax:
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    .replace(/\bimport type\b/g, 'import')
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    .replace(/\btype ([_a-zA-Z]+)( *[,\n}])/g, '$1$2')
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    // We aren't JS anymore:
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    .replace(/\/\/ @flow/, '')
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    .replace(/'(\.[^']*)\.js'/, "'$1'")
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  return output
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}

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module.exports = function jsToTs(js) {
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	var ts = babel.transform(js, {
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		plugins: (0, _loadPlugins.generatePlugins)()
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	}).code;
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	return ensureLastNewline(compactMultipleNewlines(ts));
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};