Solution:

1. Single quoted strings will display things almost completely "as is." Variables and most escape sequences will not be interpreted. The exception is that to display a literal single quote, you can escape it with a back slash \', and to display a back slash, you can escape it with another backslash \\ (So yes, even single quoted strings are parsed).
2. Double quote strings will display a host of escaped characters (including some regexes), and variables in the strings will be evaluated. An important point here is that you can use curly braces to isolate the name of the variable you want evaluated. For example let's say you have the variable $type and you what to echo "The $types are" That will look for the variable $types. To get around this use echo "The {$type}s are" You can put the left brace before or after the dollar sign. Take a look at string parsing to see how to use array variables and such.
3. Heredoc string syntax works like double quoted strings. It starts with <<<. After this operator, an identifier is provided, then a newline. The string itself follows, and then the same identifier again to close the quotation. You don't need to escape quotes in this syntax.
4. Nowdoc (since PHP 5.3.0) string syntax works essentially like single quoted strings. The difference is that not even single quotes or backslashes have to be escaped. A nowdoc is identified with the same <<< sequence used for heredocs, but the identifier which follows is enclosed in single quotes, e.g. <<<'EOT'. No parsing is done in nowdoc.

++[[]][+[]]+[+[]]

Solution:

++[[]][+[]]
+
[+[]]

++[[]][0]
+
[0]

• [[]][0] returns the inner array ([]). Due to references it's wrong to say [[]][0] === [], but let's call the inner array A to avoid the wrong notation.
• ++[[]][0] == A + 1, since ++ means 'increment by one'.
• ++[[]][0] === +(A + 1); in other words, it will always be a number (+1 does not necessarily return a number, whereas ++ always does - thanks to Tim Down for pointing this out).

+([] + 1)
+
[0]

• +([] + 1) === +("" + 1), and
• +("" + 1) === +("1"), and
• +("1") === 1

1
+
[0]

1
+
"0"

=== "10" // Yay!

     11.4.6 Unary + Operator

The unary + operator converts its operand to Number type.

The production UnaryExpression : + UnaryExpression is evaluated as follows:

1. Let expr be the result of evaluating UnaryExpression.
2. Return ToNumber(GetValue(expr)).

       Object

Apply the following steps:

1. Let primValue be ToPrimitive(input argument, hint String).
2. Return ToString(primValue).

         Object

Return a default value for the Object. The default value of an object is retrieved by calling the [[DefaultValue]] internal method of the object, passing the optional hint PreferredType. The behaviour of the [[DefaultValue]] internal method is defined by this specification for all native ECMAScript objects in 8.12.8.

        8.12.8 [[DefaultValue]] (hint)

When the [[DefaultValue]] internal method of O is called with hint String, the following steps are taken:

1. Let toString be the result of calling the [[Get]] internal method of object O with argument "toString".
2. If IsCallable(toString) is true then,

a. Let str be the result of calling the [[Call]] internal method of toString, with O as the this value and an empty argument list.

b. If str is a primitive value, return str.

        15.4.4.2 Array.prototype.toString ( )

When the toString method is called, the following steps are taken:

1. Let array be the result of calling ToObject on the this value.
2. Let func be the result of calling the [[Get]] internal method of array with argument "join".
3. If IsCallable(func) is false, then let func be the standard built-in method Object.prototype.toString (15.2.4.2).
4. Return the result of calling the [[Call]] internal method of func providing array as the this value and an empty arguments list.

       11.4.6 Unary + Operator

The unary + operator converts its operand to Number type.

The production UnaryExpression : + UnaryExpression is evaluated as follows:

1. Let expr be the result of evaluating UnaryExpression.
2. Return ToNumber(GetValue(expr)).

     The MV of StringNumericLiteral ::: [empty] is 0.

      Problem at line 1 character 1: Missing "use strict" statement.

Solution:

       Strict Mode is a new feature in ECMAScript 5 that allows you to place a program, or a    function, in a "strict" operating context. This strict context prevents certain actions from being taken and throws more exceptions.

        Strict mode helps out in a couple ways:

• It catches some common coding bloopers, throwing exceptions.
• It prevents, or throws errors, when relatively "unsafe" actions are taken (such as gaining access to the global object).
• It disables features that are confusing or poorly thought out.

  // Non-strict code...

   (function(){
  "use strict";

  // Define your library strictly...
  })();

  // Non-strict code... 

var functionOne = function() {
    // Some code
};

function functionTwo() {
    // Some code
}

Answer :

// TypeError: undefined is not a function
functionOne();

var functionOne = function() {
  console.log("Hello!");
};

// Outputs: "Hello!"
functionTwo();

function functionTwo() {
  console.log("Hello!");
}

if (test) {
   // Error or misbehavior
   function functionThree() { doSomething(); }
}