/**
 * @name Omitted array element
 * @description Omitted elements in array literals are easy to miss and should not be used.
 * @kind problem
 * @problem.severity recommendation
 * @id js/omitted-array-element
 * @tags maintainability
 *       readability
 *       language-features
 * @precision low
 */

import javascript

/**
 * An initial omitted element in an array expression.
 *
 * This is represented by the corresponding array expression, with a special
 * `hasLocationInfo` implementation that assigns it a location covering the
 * first omitted array element.
 */
class OmittedArrayElement extends ArrayExpr {
  int idx;

  OmittedArrayElement() { idx = min(int i | this.elementIsOmitted(i)) }

  /**
   * Holds if this element is at the specified location.
   * The location spans column `startcolumn` of line `startline` to
   * column `endcolumn` of line `endline` in file `filepath`.
   * For more information, see
   * [Locations](https://codeql.github.com/docs/writing-codeql-queries/providing-locations-in-codeql-queries/).
   */
  predicate hasLocationInfo(
    string filepath, int startline, int startcolumn, int endline, int endcolumn
  ) {
    exists(Token pre, Location before, Location after |
      idx = 0 and pre = this.getFirstToken()
      or
      pre = this.getElement(idx - 1).getLastToken().getNextToken()
    |
      before = pre.getLocation() and
      after = pre.getNextToken().getLocation() and
      before.hasLocationInfo(filepath, startline, startcolumn, _, _) and
      after.hasLocationInfo(_, _, _, endline, endcolumn)
    )
  }
}

from OmittedArrayElement ae
select ae, "Avoid omitted array elements."