help about: "copying value lets Flow guarantee the pointer isn't modified elsewhere"

[ghost]

I would be obliged if one explain AddUniqueOp.applyTo comment:
https://github.com/parse-community/Parse-SDK-JS/blob/master/src/ParseOp.js#L199

applyTo(value: mixed | Array<mixed>): Array<mixed> {
    if (value == null) {
            return this._value || [];
    }
    if (Array.isArray(value)) {
      // copying value lets Flow guarantee the pointer isn't modified elsewhere
      var valueCopy = value;
      var toAdd = [];
      this._value.forEach((v) => {
        if (v instanceof ParseObject) {
          if (!arrayContainsObject(valueCopy, v)) {
            toAdd.push(v);
          }
        } else {
          if (valueCopy.indexOf(v) < 0) {
            toAdd.push(v);
          }
        }
      })
      return value.concat(toAdd);
    }
    throw new Error('Cannot add elements to a non-array value');
}

why we need valueCopy reference?

var valueCopy = value;

and why we need copy technique?

thanks

[stale]

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