how to escape tag name in document selector inside javascript onclick return confirm which also contains php values

i wanted to display another tag value inside an onclick return confirm message that also contained php code, so it was a challenge to figure out how to escape all the single quote and double quotes variables. this one gave me a run for my money, but here's the example code that finally works:

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<input type="text" name="newString">
<button type="submit" onclick="return confirm('Change [ <?= $this->oldString ?> ] to [ '+document.getElementsByName('newString')[0].value+' ]?')">change string</button>
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