[Mask input phone] pure JS #mask #js

index.js

window.addEventListener("DOMContentLoaded", function() {
  [].forEach.call( document.querySelectorAll('input.wpcf7-tel'), function(input) {
    var keyCode;
    function mask(event) {
      event.keyCode && (keyCode = event.keyCode);
      var pos = this.selectionStart;
      if (pos < 3) event.preventDefault();
      var matrix = "+7 (___) ___-__-__",
          i = 0,
          def = matrix.replace(/\D/g, ""),
          val = this.value.replace(/\D/g, ""),
          new_value = matrix.replace(/[_\d]/g, function(a) {
            return i < val.length ? val.charAt(i++) || def.charAt(i) : a
          });
      i = new_value.indexOf("_");
      if (i != -1) {
        i < 5 && (i = 3);
        new_value = new_value.slice(0, i)
      }
      var reg = matrix.substr(0, this.value.length).replace(/_+/g,
                                                            function(a) {
        return "\\d{1," + a.length + "}"
      }).replace(/[+()]/g, "\\$&");
      reg = new RegExp("^" + reg + "$");
      if (!reg.test(this.value) || this.value.length < 5 || keyCode > 47 && keyCode < 58) this.value = new_value;
      if (event.type == "blur" && this.value.length < 5)  this.value = ""
    }

    input.addEventListener("input", mask, false);
    input.addEventListener("focus", mask, false);
    input.addEventListener("blur", mask, false);
    input.addEventListener("keydown", mask, false)

  });

});