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    <p>if property we want to check in object whether that it exists or not, even if its undefined.</p> <p>we will use one of these: <strong>'prop' in obj</strong>(to check for properties from prototype chain) or <strong>obj.hasOwnProperty('prop')</strong> </p> <p>we need to use methods above to check if property exists as accessing property that has not been declared in object will also return undefined.</p> <pre><code>var o={}; o.c=undefined; o.c===undefined; //is true o.a===undefined; //is true as well even though c exists while a doesn't </code></pre> <p>commonly not a problem as nobody really declare undefined properties much, but when do so do it like this. </p> <pre><code>o.c=''; //when it can be string or o.c=null; //to clearly indicate that its nothing. then o.c === undefined will return false! note!!! null == undefined //true while null === undefined //false that's why use three equals to test </code></pre> <p>For variables not declared and are not inside object. When accessed The compiler will return (reference)error. If it doesn't it means its being treated as a global property, <strong>window object</strong> property, and was not declared, at least in all parent scope, so it will be undefined just as <code>o.a</code> was at top. it will become <code>window.prop</code>.</p> <pre><code>so x; //error but x=3; //no error assumed to be global object. just like o.abcd = 3; would... make(declare) a property abcd in object o valued(assigned) 3 all at once. </code></pre> <p>To avoid properties to become a global variable we use <strong>var keyword inside function</strong>, like this <code>var k;</code></p> <p>One thing you can do about this catch the reference error when throw for a variable that doesn't exist and are thought to be a variable itself. </p> <pre><code>try { x } catch(e){//code to run when x is not declared let alone defined.} </code></pre>
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