JSFiddle - React, Tailwind, and code Playground

HTML

<input id="a" type="checkbox" />
<input type="text" id="foo[1]" name="foo[1]" />
<input type="text" id="foo[2]" name="foo[2]" />
<input type="text" id="foo[3]" name="foo[3]" />
<input type="text" id="foo[4]" name="foo[4]" />
<input type="text" id="foo[5]" name="foo[5]" />

JavaScript

$("#a").click(function() {
    var first = $('[id=foo[1]]');
    var next = first.nextAll(':text').filter(function() {
        console.log(this.id);
        return /foo\[\d+\]/.test(this.id);
    });
    if($(this).is(':checked')) {
        next.val(first.val());
    }
    else {
        next.val('');
    }   
});