[Solved]-Add value from inputs to array if not exists-Reactjs

You need to append the inputObject to the sendTransferObj array in the current state instead of appending it to a new empty array (which will cause the overwriting). You can do this using the spread syntax

handleChange(transId: string, event: React.ChangeEvent<HTMLInputElement>) {
    let inputObject = [{ id: transId, value: event.target.value }]
    const index = inputObject.findIndex(inputObject => inputObject.id != transId)

    this.setState({
        ...this.state,
        sendTransferObj: index === -1 ? [
          ...this.state.sendTransferObj,
          inputObject
        ] : this.state.sendTransferObj
    });
}

This won't update the field when the inputObject is found, though. I would recommend storing the values in an object with the ID as keys instead (and formatting it into the desired format when requesting the API). This way, you don't have to iterate through the array to find the matching object.

constructor(props: Props) {
    super(props);
    this.state = {
        sendTransferObj: {}
    };
    this.handleChange = this.handleChange.bind(this);
}

...

handleChange(transId: string, event: React.ChangeEvent<HTMLInputElement>) {
      this.setState({
          ...this.state,
          sendTransferObj: {
            ...sendTransferObj,
            [transId]: event.target.value
          }
      }); 
  }