null vs undefined

Similarities
Both are primitives
Both are falsy:
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Boolean(undefined) //-> false
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Boolean(null) //-> false
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Differences
undefined means that a variable has not been declared, or it has been declared but has not yet been assigned a value.
null is an assignment value that means “no value”. Javascript itself never sets a value to null
Difference in typeof:
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typeof null; //-> "object"
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typeof undefined; //-> "undefined"
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undefined is not a valid JSON while null is.
Tips
Check if a variable is null:
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variable === null;
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Check if a variable is undefined:
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typeof variable === 'undefined';
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​
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// ⬇️ or this, but will throw ReferenceError if variable wasn't defined
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variable === undefined;
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Comparison (Equality returns true and identity returns false) :
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undefined == null; //-> true
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​
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undefined === null; //-> false
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