- Units Conversion. This manual describes the units command for units conversion and how you can use it as a powerful scientific calculator that keeps track of units. This is Edition 2.16 of The Units Conversion Manual for units Version 2.18.
- Table Size We can set the size of the table based of pixels or percentage. First we will see how to set size based on pixels. The attribute width and height will be used.
You should be able to get a 5x7 inch print made from a 1024x768 pixel file. However if you want the photo to fill the 7 inch width of the print you will loose a few pixels at the top and bottom with only 731 pixels being printed. If you do not want to loose any pixels, you will end up with white space at the left and right sides.
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Number() converts to a Number, String() converts to a String, Boolean() converts to a Boolean.
JavaScript Data Types
In JavaScript there are 5 different data types that can contain values:
stringnumberbooleanobjectfunction
There are 6 types of objects:
ObjectDateArrayStringNumberBoolean
And 2 data types that cannot contain values:
nullundefined
The typeof Operator
You can use the typeof operator to find the data type of a JavaScript variable.
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Example
typeof 3.14 // Returns 'number'
typeof NaN // Returns 'number'
typeof false // Returns 'boolean'
typeof [1,2,3,4] // Returns 'object'
typeof {name:'John', age:34} // Returns 'object'
typeof new Date() // Returns 'object'
typeof function () {} // Returns 'function'
typeof myCar // Returns 'undefined' *
typeof null // Returns 'object'
Please observe:
- The data type of NaN is number
- The data type of an array is object
- The data type of a date is object
- The data type of null is object
- The data type of an undefined variable is undefined *
- The data type of a variable that has not been assigned a value is also undefined *
You cannot use typeof to determine if a JavaScript object is an array (or a date).
The Data Type of typeof
The typeofoperator is not a variable. It is an operator. Operators ( + - * / ) do not have any data type.
But, the typeof operator always returns a string (containing the type of the operand).
The constructor Property
The constructor property returns the constructor function for all JavaScript variables.
Example
(3.14).constructor // Returns function Number() {[native code]}
false.constructor // Returns function Boolean() {[native code]}
[1,2,3,4].constructor // Returns function Array() {[native code]}
{name:'John',age:34}.constructor // Returns function Object() {[native code]}
new Date().constructor // Returns function Date() {[native code]}
function () {}.constructor // Returns function Function(){[native code]}
You can check the constructor property to find out if an object is an Array (contains the word 'Array'):
Example
return myArray.constructor.toString().indexOf('Array') > -1;
}
Or even simpler, you can check if the object is an Array function:
Example
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return myArray.constructor Array;
}
You can check the constructor property to find out if an object is a Date (contains the word 'Date'):
Example
return myDate.constructor.toString().indexOf('Date') > -1;
}
Or even simpler, you can check if the object is a Date function:
Example
return myDate.constructor Date;
}
JavaScript Type Conversion
JavaScript variables can be converted to a new variable and another data type:
- By the use of a JavaScript function
- Automatically by JavaScript itself
Converting Numbers to Strings
The global method String() can convert numbers to strings.
It can be used on any type of numbers, literals, variables, or expressions:
Example
String(123) // returns a string from a number literal 123
String(100 + 23) // returns a string from a number from an expression
The Number method toString() does the same.
Example
(123).toString()
(100 + 23).toString()
In the chapter Number Methods, you will find more methods that can be used to convert numbers to strings:
| Method | Description |
|---|---|
| toExponential() | Returns a string, with a number rounded and written using exponential notation. |
| toFixed() | Returns a string, with a number rounded and written with a specified number of decimals. |
| toPrecision() | Returns a string, with a number written with a specified length |
Converting Booleans to Strings
The global method String() can convert booleans to strings.
String(true) // returns 'true'
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The Boolean method toString() does the same.
true.toString() // returns 'true'
Converting Dates to Strings
The global method String() can convert dates to strings.
The Date method toString() does the same.
Example
In the chapter Date Methods, you will find more methods that can be used to convert dates to strings:
| Method | Description |
|---|---|
| getDate() | Get the day as a number (1-31) |
| getDay() | Get the weekday a number (0-6) |
| getFullYear() | Get the four digit year (yyyy) |
| getHours() | Get the hour (0-23) |
| getMilliseconds() | Get the milliseconds (0-999) |
| getMinutes() | Get the minutes (0-59) |
| getMonth() | Get the month (0-11) |
| getSeconds() | Get the seconds (0-59) |
| getTime() | Get the time (milliseconds since January 1, 1970) |
Converting Strings to Numbers
The global method Number() can convert strings to numbers.
Strings containing numbers (like '3.14') convert to numbers (like 3.14).
Empty strings convert to 0.
Anything else converts to NaN (Not a Number).
Number(' ') // returns 0
Number(') // returns 0
Number('99 88') // returns NaN
In the chapter Number Methods, you will find more methods that can be used to convert strings to numbers:
| Method | Description |
|---|---|
| parseFloat() | Parses a string and returns a floating point number |
| parseInt() | Parses a string and returns an integer |
The Unary + Operator
The unary + operator can be used to convert a variable to a number:
Example
var x = + y; // x is a number
If the variable cannot be converted, it will still become a number, but with the value NaN (Not a Number):
Example
var x = + y; // x is a number (NaN)
Converting Booleans to Numbers
The global method Number() can also convert booleans to numbers.
Number(true) // returns 1
Converting Dates to Numbers
The global method Number() can be used to convert dates to numbers.
Number(d) // returns 1404568027739
The date method getTime() does the same.
d.getTime() // returns 1404568027739
Automatic Type Conversion
When JavaScript tries to operate on a 'wrong' data type, it will try to convert the value to a 'right' type.
The result is not always what you expect:
'5' + null // returns '5null' because null is converted to 'null'
'5' + 2 // returns '52' because 2 is converted to '2'
'5' - 2 // returns 3 because '5' is converted to 5
'5' * '2' // returns 10 because '5' and '2' are converted to 5 and 2
Automatic String Conversion
JavaScript automatically calls the variable's toString() function when you try to 'output' an object or a variable:
// if myVar = {name:'Fjohn'} // toString converts to '[object Object]'
// if myVar = [1,2,3,4] // toString converts to '1,2,3,4'
// if myVar = new Date() // toString converts to 'Fri Jul 18 2014 09:08:55 GMT+0200'
Numbers and booleans are also converted, but this is not very visible:
// if myVar = true // toString converts to 'true'
// if myVar = false // toString converts to 'false'
JavaScript Type Conversion Table
This table shows the result of converting different JavaScript values to Number, String, and Boolean:
| Original Value | Converted to Number | Converted to String | Converted to Boolean | Try it |
|---|---|---|---|---|
| false | 0 | 'false' | false | Try it » |
| true | 1 | 'true' | true | Try it » |
| 0 | 0 | '0' | false | Try it » |
| 1 | 1 | '1' | true | Try it » |
| '0' | 0 | '0' | true | Try it » |
| '000' | 0 | '000' | true | Try it » |
| '1' | 1 | '1' | true | Try it » |
| NaN | NaN | 'NaN' | false | Try it » |
| Infinity | Infinity | 'Infinity' | true | Try it » |
| -Infinity | -Infinity | '-Infinity' | true | Try it » |
| ' | 0 | ' | false | Try it » |
| '20' | 20 | '20' | true | Try it » |
| 'twenty' | NaN | 'twenty' | true | Try it » |
| [ ] | 0 | ' | true | Try it » |
| [20] | 20 | '20' | true | Try it » |
| [10,20] | NaN | '10,20' | true | Try it » |
| ['twenty'] | NaN | 'twenty' | true | Try it » |
| ['ten','twenty'] | NaN | 'ten,twenty' | true | Try it » |
| function(){} | NaN | 'function(){}' | true | Try it » |
| { } | NaN | '[object Object]' | true | Try it » |
| null | 0 | 'null' | false | Try it » |
| undefined | NaN | 'undefined' | false | Try it » |
Values in quotes indicate string values.
Red values indicate values (some) programmers might not expect.
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