eslint-plugin-babel/rules/no-await-in-loop.js

/**
 * @fileoverview Rule to disallow uses of await inside of loops.
 * @author Nat Mote
 */
"use strict";

// Node types which are considered loops.
var loopTypes = {
  'ForStatement': true,
  'ForOfStatement': true,
  'ForInStatement': true,
  'WhileStatement': true,
  'DoWhileStatement': true,
};

// Node types at which we should stop looking for loops. For example, it is fine to declare an async
// function within a loop, and use await inside of that.
var boundaryTypes = {
  'FunctionDeclaration': true,
  'FunctionExpression': true,
  'ArrowFunctionExpression': true,
};

module.exports = function(context) {
  return {
    AwaitExpression(node) {
      var ancestors = context.getAncestors();
      // Reverse so that we can traverse from the deepest node upwards.
      ancestors.reverse();
      // Create a set of all the ancestors plus this node so that we can check
      // if this use of await appears in the body of the loop as opposed to
      // the right-hand side of a for...of, for example.
      //
      // Implement the set with an Array since there are likely to be very few
      // elements. An Object would not be appropriate since the elements are
      // not strings.
      var ancestorSet = [].concat(ancestors, [node]);
      var ancestorSetHas = function(element) {
        return ancestorSet.indexOf(element) !== -1;
      }
      for (var i = 0; i < ancestors.length; i++) {
        var ancestor = ancestors[i];
        if (boundaryTypes.hasOwnProperty(ancestor.type)) {
          // Short-circuit out if we encounter a boundary type. Loops above
          // this do not matter.
          return;
        }
        if (loopTypes.hasOwnProperty(ancestor.type)) {
          // Only report if we are actually in the body or another part that gets executed on
          // every iteration.
          if (
            ancestorSetHas(ancestor.body) ||
            ancestorSetHas(ancestor.test) ||
            ancestorSetHas(ancestor.update)
          ) {
            context.report(
              node,
              'Avoid using await inside a loop. Consider refactoring to use Promise.all. If ' +
              'you are sure you want to do this, add `// eslint-disable-line ' +
              context.id + '` at the end of this line.'
            );
            return;
          }
        }
      }
    },
  };
}