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Closure Douglas Crockford
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Block Scope { let a; { let b; … a … … b … } … a … }
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Function Scope function green() { let a; function yellow() { let b; … a … … b … } … a … }
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Function Scope function green() { let a; function yellow() { let b; … a … … b … } … a … } a
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Function Scope function green() { let a; function yellow() { let b; … a … … b … } … a … } a b
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Lisp [1958] dynamic scope nested functions function values Algol 60 [1960] lexical scope nested functions functions are not values C [1972] lexical scope functions cannot nest functions are values
![Lisp [1958] dynamic scope nested functions function values Algol 60 [1960] lexical scope nested functions functions are not values C [1972] lexical scope functions cannot nest functions are values](http://images.slideplayer.com/11/3170229/slides/slide_6.jpg "Lisp [1958] dynamic scope nested functions function values Algol 60 [1960] lexical scope nested functions functions are not values C [1972] lexical scope functions cannot nest functions are values")
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Inner survives the outer function green() { let a; return function yellow() { let b; … a … … b … }; … a … }
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Global var names = ['zero', 'one', 'two', 'three', 'four', 'five', 'six', 'seven', 'eight', 'nine']; var digit_name = function (n) { return names[n]; }; alert(digit_name(3)); // 'three'
![Global var names = [ zero , one , two , three , four , five , six , seven , eight , nine ]; var digit_name = function (n) { return names[n]; }; alert(digit_name(3)); // three](http://images.slideplayer.com/11/3170229/slides/slide_8.jpg "Global var names = [ zero , one , two , three , four , five , six , seven , eight , nine ]; var digit_name = function (n) { return names[n]; }; alert(digit_name(3)); // three")
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Slow var digit_name = function (n) { var names = ['zero', 'one', 'two', 'three', 'four', 'five', 'six', 'seven', 'eight', 'nine']; return names[n]; }; alert(digit_name(3)); // 'three'
![Slow var digit_name = function (n) { var names = [ zero , one , two , three , four , five , six , seven , eight , nine ]; return names[n]; }; alert(digit_name(3)); // three](http://images.slideplayer.com/11/3170229/slides/slide_9.jpg "Slow var digit_name = function (n) { var names = [ zero , one , two , three , four , five , six , seven , eight , nine ]; return names[n]; }; alert(digit_name(3)); // three")
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Closure var digit_name = (function () { var names = ['zero', 'one', 'two', 'three', 'four', 'five', 'six', 'seven', 'eight', 'nine']; return function (n) { return names[n]; }; }()); alert(digit_name(3)); // 'three'
![Closure var digit_name = (function () { var names = [ zero , one , two , three , four , five , six , seven , eight , nine ]; return function (n) { return names[n]; }; }()); alert(digit_name(3)); // three](http://images.slideplayer.com/11/3170229/slides/slide_10.jpg "Closure var digit_name = (function () { var names = [ zero , one , two , three , four , five , six , seven , eight , nine ]; return function (n) { return names[n]; }; }()); alert(digit_name(3)); // three")
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Start Over var names = ['zero', 'one', 'two', 'three', 'four', 'five', 'six', 'seven', 'eight', 'nine']; var digit_name = function (n) { return names[n]; }; alert(digit_name(3)); // 'three'
![Start Over var names = [ zero , one , two , three , four , five , six , seven , eight , nine ]; var digit_name = function (n) { return names[n]; }; alert(digit_name(3)); // three](http://images.slideplayer.com/11/3170229/slides/slide_11.jpg "Start Over var names = [ zero , one , two , three , four , five , six , seven , eight , nine ]; var digit_name = function (n) { return names[n]; }; alert(digit_name(3)); // three")
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Immediate function returns a function var names = ['zero', 'one', 'two', 'three', 'four', 'five', 'six', 'seven', 'eight', 'nine']; var digit_name = (function () { return function (n) { return names[n]; }; }()); alert(digit_name(3)); // 'three'
![Immediate function returns a function var names = [ zero , one , two , three , four , five , six , seven , eight , nine ]; var digit_name = (function () { return function (n) { return names[n]; }; }()); alert(digit_name(3)); // three](http://images.slideplayer.com/11/3170229/slides/slide_12.jpg "Immediate function returns a function var names = [ zero , one , two , three , four , five , six , seven , eight , nine ]; var digit_name = (function () { return function (n) { return names[n]; }; }()); alert(digit_name(3)); // three")
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Closure var digit_name = (function () { var names = ['zero', 'one', 'two', 'three', 'four', 'five', 'six', 'seven', 'eight', 'nine']; return function (n) { return names[n]; }; }()); alert(digit_name(3)); // 'three'
![Closure var digit_name = (function () { var names = [ zero , one , two , three , four , five , six , seven , eight , nine ]; return function (n) { return names[n]; }; }()); alert(digit_name(3)); // three](http://images.slideplayer.com/11/3170229/slides/slide_13.jpg "Closure var digit_name = (function () { var names = [ zero , one , two , three , four , five , six , seven , eight , nine ]; return function (n) { return names[n]; }; }()); alert(digit_name(3)); // three")
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