JavaScript gotchas

JavaScript gotchas

• 1.
  JavaScript Gotchas Recommendations Pierre Nallet www.javascriptgotchas.com
• 2.
  A good return return; return x; return { x:2 }; Always terminate statements with ; Start the returned expression on the same line
• 3.
  switch and break switch(dayOfWeek) { case0: ... break; case 1: case 2: return something; ... default: throw new Error("invalid arg"); } End your case statements with break ... … or return Consider adding a default case
• 4.
  Don't let blocksblock you if (condition) doSomething(); doSomethingElse(); Consider one line if statements if (condition){ doSomething(); } doSomethingElse(); Consider adding braces
• 5.
  The old parseInt parseInt("010",10) // 10 parseFloat("010") // 10 Number("010") // 10 Always specify radix in parseInt Consider using parseFloat or Number
• 6.
  Number imprecision for(var i= 0; i < 10; i = i + 0.1) { ... } Terminate loops with inequalities, not equalities var noTruth = 0.3 === 0.1 + 0.2 var truth = close(0.3, 0.1 + 0.2); Remember number precision problems Consider using tolerance
• 7.
  Not a numberis a number Remember: isNaN not the same as Number.isNaN if(isFinite(n)){ // all is good } Use isFinite to protect against non finite values if(isNaN(n)){ // Could be infinite } Consider using isNan Do not write n === NaN
• 8.
  A bad date varday = date.getDate(); // day of month var month = date.getMonth() + 1 // month, o Add 1 since months are zero based Use getDate to get day of month, not getDay
• 9.
  + 2 + 3 "hello" + name "the number is " + num // OK 1 + someString // problem Add items of the same type Be careful when adding different types Don't use + on objects
• 10.
  forEach for everyone array.forEach(function(item) { // ... } ); use forEach for array iteration Do not use for loops on arrays
• 11.
  push your array myArray.push(post); if(index < 0 || index >= posts.length) { throw new Error('Invalid index'); } posts[index] = post; Use push to add new items to an array. Verify array bounds before writing to arrays via an index
• 12.
  splice, don't delete vara = [1, 2, 3, 4, 5]; a.splice(2, 1); // a contains 1,2,4,5 Use array.splice, to remove items from array Do not delete on arrays
• 13.
  Sorting out array.sort varnumbers = [1, 5, 10]; // [1, 5, 10] numbers.sort(ascending); // [1, 5, 10] var ascending = function(x, y){ if (x > y) return 1; else if (x < y) return -1; return 0; } Use sort functions for sorting arrays You don't have to use a sort function if the array contains only strings
• 14.
  Arrays vs. dictionaries vardict = {}; dict.name = value1; dict.['age'] = value2; Use empty JavaScript objects for dictionaries, not arrays var array = [1,2,3] ; array[0]; Use only numbers to access array items
• 15.
  All equals arenot equal if (userChoice == "Purchase") { // should purchase } Always be on guard against a missed = sign. Equality expression is == or ===, not =
• 16.
  More equality isbetter Use == and != only when you are sure you compare with the same type if (x === 0){ //do something } Prefer === and !== over == and !=
• 17.
  null is notundefined typeof x === "undefined" a.b === void 0 Checks for undefined Consider setting valid variables to null x == null Checks for null or undefined var p = {name:'Alice', age: null};
• 18.
  Missing the argument functionaddToCart(productName, quantity){ if(quantity === void 0) quantity = 1; … } Consider default parameters
• 19.
  var makes adifference var setup = function (mode) { var unload = mode === "Production"; // ... } Always use var when declaring variables. var setup = function (mode) { window.unload = // ... } Consider using window. when accessing global variables
• 20.
  namespaces var app =app || {} app.core = app.core || {} app.core.log = function() {} // ... When (potentially) reinitializing a variable, use the || syntax.
• 21.
  this and that functioncomputeSum(array){ this.total = 0; var that = this; array.forEach(function(item){ that.total += item; }); } Capture this before reaching callbacks
• 22.
  Stay up todate www.javascriptgotchas.com