Below image summarise the lifecycle methods exposed by react component and the order in which lifecycle methods are being called by React. React 16 was a major release for ReactJS. With this version react has marked some of the lifecycle methods like componentWillRecieveProps, ComponentWillUpdate as deprecated. In a later release of React, the framework will be removing these methods. In place of deprecated methods, they added a few new lifecycle methods. The above picture shows the lifecycle method of a component. We can divide the lifecycle methods of the component into three categories.

Though we can implement in both functional and class-based way, there are few fundamental differences between the two of them:

here are two ways of writing a react component Example of functional component Example of class-based component cases are known as a presentational or dumb component. The functional component gets information via props from its parent component and renders the information. You can see that the functional component accepts props as an argument. ES6 arrow function also is used to define a functional component. The functional component doesn’t store any state and also doesn’t override any lifecycle method of React component. The class-based component has the ability to store state within the component and based on its state value behave differently. The latest version of React 16.8 has included a new feature called React hooks by which we can add state in a functional component.

Though most of the CSS properties works in the same way in React native still there are few differences in CSS property values between a browser and React native: Apart from the above difference, most of the CSS properties work exactly the same as they work in a browser.

By default, the main axis is the vertical axis and cross axis is the horizontal axis in React native. Since justifyContent and alignItems property works based on the main axis and cross axis, so justifyContent will align flex items vertically and alignItems will layout flex item horizontally. This default value of the main axis and cross axis can be changed by changing flexDirection property. If we set flexDirection to row in the flex container then the main axis will become horizontal axis and cross axis will become vertical axis.  On changing default behaviour via flexDirection CSS property, the behaviour of alignItems and justifyContent will also get switched. The Concept of flexDirection is also present on the web. Flexbox is implemented as a single direction layout technique and this direction is managed by flexDirection CSS property. Apart from row and column, the flexDirection property has two more values: row-reverse and column-reverse. As the name suggests, the direction will remain horizontal and vertical but the direction will get changed. Transitions are an important feature of mobile devices. So, this is one of the most frequently asked React Native interview questions and answers for senior developer in recent times.

You can think of alignItems as justifyContent behaviour for cross axis. Cross-axis in case if React native is the horizontal axis. CSS alignItems property sets the alignSelf value on all direct children as a group. In Flexbox, it controls the alignment of items on the cross axis. By default, the cross axis is a horizontal axis in case of React native. We can use this property to layout elements in the flex container. The alignItems property supports following values: stretch|center|flex-start|flex-end|baseline|initial|inherit;  Let me explain alignItems values:

JustifyContent property aligns the flexible container’s items when the items do not use all available space on the main axis. By default, the main axis is a vertical axis in case of React native. Which means justifyContent property aligns child elements of flex parent vertically in React native.  We can use this property to layout elements in a flex container. JustifyContent supports the following values: flex-start|flex-end|centre|space-between|space-around|initial|inherit; It also apply some control over the alignment of items when they overflow the line. Let me explain JustifyContent’s values:

CSS Flexbox is used to design a responsive layout easily without using float or position CSS property. Float and position values were used to build any type of UI which are not very easy. Flexbox is added in CSS3. Flexbox is designed to provide a consistent layout on different screen sizes. You will normally use a combination of flexDirection, alignItems, and justify-content to achieve the right layout.  There are few differences in default values of few flex based CSS properties between React Native and on the browser. The default value of flexDirection is a row in the web but in case of React native, its default value is a column. Also, flex parameter only supports a single number in React native. Flexbox alignItems has few more values like flex-start which start aligning element from start and opposite of this is flex-end which places the first child element at the end. Similar to alignItems, justifyContents also have values which behaves where much similar to flexbox behaviour in a web browser.

By default, the View component in React Native has its display property set to flex. Flex is a CSS property used to expand and shrink components dynamically based on available space. Setting flex: 1 on a component makes it take up all the available space. If a parent element has flex: 1, its child elements will evenly distribute the available space among themselves.  The width of a child element can be adjusted by assigning it a higher flex value; the larger the value, the more space the component will take relative to its siblings. However, for a component to expand, its parent must have dimensions greater than zero. If the parent element’s dimensions are not set, the flex property will not work, and the element will not be visible on the UI.  In addition to the flex property, align-items and justify-content CSS properties are also used to design mobile UIs in React Native. 

React native follows the box-model concept of CSS. The size of the element is calculated based on the size of content, padding, border, margin. The simplest way to set the size of an element is to set width and height CSS property for an element. All dimensions in React Native is unitless and represent density-independent pixels. By setting fixed height and width, the element will look exactly the same size on different screen sizes. But there is an instance where you want to give the width and height of an element in percentage.  Directly use of percentage is not supported in React native but React native does give a dimension module which can be used to give width in percentage. Dimension module gives the width and height of the mobile device. This information can be used to set the style of an element in runtime. Below is the example of how to use Dimension module from React native: But the simplest way is by setting width and height CSS for an element.